Typescript cast any to number

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Typescript cast any to number

TypeScript allows you to override its inferred and analyzed view of types in any way you want to. This is done by a mechanism called "type assertion". TypeScript's type assertion is purely you telling the compiler that you know about the types better than it does, and that it should not second guess you.A common use case for type assertion is when you are porting over code from JavaScript to TypeScript. For example consider the following pattern:var foo = {};foo.bar = 123; foo.bas = 'hello'; Here the code errors because the inferred type of foo is {} i.e. an object with zero properties. Therefore you are not allowed to add bar or bas to it. You can fix this simply by a type assertion as Foo:interface Foo { bar: number; bas: string;}var foo = {} as Foo;foo.bar = 123;foo.bas = 'hello';Originally the syntax that was added was . This is demonstrated below:var foo: any;var bar = foo; However, there is an ambiguity in the language grammar when using style assertions in JSX:var foo = bar;Therefore it is now recommended that you just use as foo for consistency.The reason why it's not called "type casting" is that casting generally implies some sort of runtime support. However, type assertions are purely a compile time construct and a way for you to provide hints to the compiler on how you want your code to be analyzed.In many cases assertion will allow you to easily migrate legacy code (and even copy paste other code samples into your codebase). However, you should be careful with your use of assertions. Take our original code as a sample, the compiler will not protect you from forgetting to actually add the properties you promised:interface Foo { bar: number; bas: string;}var foo = {} as Foo;Also another common thought is using an assertion as a means of providing autocomplete e.g.:interface Foo { bar: number; bas: string;}var foo = {};but the hazard here is the same, if you forget a property the compiler will not complain. It is better if you do the following:interface Foo { bar: number; bas: string;}var foo: Foo = {};In some cases you might need to create a temporary variable, but at least you will not be making (possibly false) promises and instead relying on the type inference to do the checking for you.The type assertion, despite being a bit unsafe as we've shown, is not completely open season. E.g. the following is a very valid use case (e.g. the user thinks the event passed in will be a more specific case of an event) and the type assertion works as expected:function handler (event: Event) { let mouseEvent = event as MouseEvent;}However, the following is most likely an error and TypeScript will complain as shown despite the user's type assertion:function handler(event: Event) { let element = event as HTMLElement; }If you still want that Type, you can use a double assertion, but first asserting to unknown (or any) which is compatible with all types and therefore the compiler no longer complains:function handler(event: Event) { let element = event as unknown as HTMLElement; }Basically, the assertion from type S to T succeeds if either S is a subtype of T or T is a subtype of S. This is to provide extra safety when doing type assertions ... completely wild assertions can be very unsafe and you need to use unknown (or any) to be that unsafe. one of the things that are very common in object-oriented languages is type casting. in javascript, which is a dynamic language, you don't have the concept of casting. on the other hand, typescript language, which compiles into javascript, does include casting. this post explains how to use casting in typescript. down casting in typescript typescript includes the ability to cast types. in order to cast a type in typescript you use the brackets with the relevant type to cast. in order to show how you can use casting lets take the greeter example that is included in typescript website : class greeter { element: htmlelement; span: htmlelement; timertoken: number; constructor (element: htmlelement) { this.element = element; this.element.innertext += "the time is: "; this.span = document.createelement('span'); this.element.appendchild(this.span); this.span.innertext = new date().toutcstring(); } start() { this.timertoken = setinterval(() => this.span.innertext = new date().toutcstring(), 500); } stop() { cleartimeout(this.timertoken); } } window.onload = () => { var el = document.getelementbyid('content'); var greeter = new greeter(el); greeter.start(); }; in the greeter class we expect a span which is of type htmlelement . typescript includes a type called htmlspanelement which is extends the htmlelement type. so if we change the declaration of the span field to span: htmlspanelement; you will get syntax error in the following constructor line: this.span = document.createelement('span'); the error occurs because the document.createelement function returns a htmlelement and not a htmlspanelement . in order to fix the problem you will need to use the casting in that line of code: this.span = document.createelement('span'); the will tell the typescript compiler to cast the htmlelement returned from document.createelement function into htmlspanelement . that will fix the syntax error. pay attention that the generated javascript will look the same for both of the options (the not modified and the modified greeter examples) and this is because of the dynamic nature of javascript. up casting in typescript as opposed to the previous down cast example, the option to up cast a type is straight forward in typescript. for example, the following code is valid: var el: htmlelement; el = new htmlspanelement(); since htmlspanelement extends the htmlelement type it is possible to set a htmlspanelement instead of htmlelement .the generated javascript for the previous code will be: var el; el = new htmlspanelement(); summary typescript includes the ability to down cast and to up cast types. you use the brackets in order to down cast a type. in up cast, you just need to know that one type is a subclass of another type. NEWBEDEVPythonJavascriptLinuxCheat sheet Announcing a new package for TypeScript called safe-cast. safe-cast is a TypeScript library for safely casting unknown data to a known type.You can find it published on NPM.The safe-cast package on NPM.What does safecast do?Let's say that you are writing a web service and you are expecting another service to pass you a blob of JSON that matches this typescript type:You might be tempted to write the following code in your service:In this example, we have performd an unsafe cast, we have said "TypeScript -- I know that you don't know what the type of rawInputData is but trust me. I'm pretty confident that it's an InputData ."This is not good. What would happen if somebody sent us this data with two mistakes:If we then tried to access title or lastUpdated on inputData then TypeScript would not complain but the program would fail at runtime.What we need is a way to assert, at runtime, that the data we have been given actually matches the type InputData . This is the problem that safe-cast solves.To use safe-cast to solve this problem, you would write the following code:And there you have it, 100% safe type casting in TypeScript with only one or two extra lines of code than what you would have written anyway. Beautiful!The general problemIn a nutshell, many programs accept input that:Is JSON (or a subset of JSON)Was created or modified outside of your program's control; by either a human or computer.May not be 100% trustworthy or, at the very least, may simply be accidentally incorrect.safe-cast validates that the data you accept is in the structure that you expect.That it what makes it so valuable.Why did you build safe-cast in the first place?I wanted the following properties to be true for a type-safe casting library:It should be trivial to add to your existing code.safe-cast only requires that you add an import and replace your "trust me Typescript" type casts with a call to the safeCast function. I did not want developers, by default, to have to write their own validation functions for this library to work.It should be performant.The first time that you call the safe-cast library and we introspect the types you want to validate performance will take a small 1?2s hit. But for every future call to cast (for most types) will take place in milliseconds.It should cover all use cases.There are many complex types that you can write in TypeScript and I wanted this validator to work against everything. To make this possible, safe-cast uses JSON Schema as it's validation tool. Your types will be converted into JSON Schema using typescript-json-schema and then the incoming data will be validated against that schema using ajv .These three properties make safe-cast unique.There must be packages that do this already? Why not use them?There are plenty of other packages out there that promise runtime validation of your types; but they almost all seem to require that you write your own validation functions instead of just deriving them for free based on your TypeScript types. Packages that fit into this category include:This is the main problem, they break the principles laid out in the top section, specifically the on that ensures that adding in safe type-casting should be a trivial operation.Where to next?The package is very new. I am going to:Try and apply it in real-world situations.Continue to write more comprehensive test cases for the package.Get TypeScript community feedback for the package.As it matures, eventually release a 1.0.0 version of the package.So, why not give safe-cast a try today and let me know what you think! As shown by other answers here, there are multiple ways to do the conversion: Number('123'); +'123'; parseInt('123'); parseFloat('123.45') I'd like to mention one more thing on parseInt though. When using parseInt, it makes sense to always pass the radix parameter. For decimal conversion, that is 10. This is the default value for the parameter, which is why it can be omitted. For binary, it's a 2 and 16 for hexadecimal. Actually, any radix between and including 2 and 36 works. parseInt('123') // 123 (don't do this) parseInt('123', 10) // 123 (much better) parseInt('1101', 2) // 13 parseInt('0xfae3', 16) // 64227 In some JS implementations, parseInt parses leading zeros as octal: Although discouraged by ECMAScript 3 and forbidden by ECMAScript 5, many implementations interpret a numeric string beginning with a leading 0 as octal. The following may have an octal result, or it may have a decimal result. Always specify a radix to avoid this unreliable behavior. -- MDN The fact that code gets clearer is a nice side effect of specifying the radix parameter. Since parseFloat only parses numeric expressions in radix 10, there's no need for a radix parameter here. More on this: Did you face the situation where you want to parse number to string or string to number in typescript?. During the development of angular/typescript based projects, I got many situations that are a result of writing this blog post. Number and String are basic Inbuilt primitive datatypes in typescript programming language. Every developer is already aware of these types. The number holds numeric values, String holds a group of characters enclosed in double or single quotes. Variable of these types hold different values. Number type does not convert to String type automatically, Therefore, Developers has to write a code to do the manual conversion. The number to String and String to Number conversion are one of the simple tasks of typescript developers. For conversion to number, String must contain only numbers. The Number to String conversion examples. There are multiple ways we can convert Number to String in javascript or typescript. Append with an empty string. toString() method string constructor Append number with an empty string. This is an easy and simple way parse number to a string. For example, given number values are 11,13, add an empty string and returns a string object. In this case, typeof method checks value type and returns primitive type as a string. In typescript, + is an operator used to append the string. The Below example converts valid and invalid values to String The below example number is valid numeric value ie 12, var a: number = 12 var str: string = a + ''; console.log(typeof (a)) // outpus number as output console.log(typeof (str)) // outpus string as output console.log(str) //outputs 12 as string value var a: number = null var str: string = a + ''; console.log(typeof (str)) // outpus string as output console.log(str) //outputs null in the form of string var a: number = undefined var str: string = a + ''; console.log(typeof (str)) // outpus string as output console.log(str) //outputs undefined in the form of string toString() method toString() method is an in-built method which every data type has. It returns string version of an object. calling toString() method on any data types returns string object. var a: number = 17 var str: string = a .toString() console.log(typeof (str)) // outpus string as output console.log(a) //outputs 12 as string value if we use null and undefined with toString() method, compiler through Cannot read property 'toString' of null error. string constructor String constructor accepts any value and returns a string. This approach useful when you are creating the string. And it is easiest way of a converted number to string type. Here is an example var a: number = 12 var stringValue = String(a); console.log(typeof(stringValue) // outpus string as output Convert String to Number example This section solves some of the below problems. How to convert string to integer? How to parse string to floating point number with decimal precision. The string can be converted to number in multiple ways. First, String must contain numeric values enclsed in single or double, Input is valid string numeric value, example is 123 and output always return 123 of type number. if string is not numeric value, All these conversion ways return NAN value. There multiple easy and short ways to parse String to number value parseInt method parseFloat method Number() constructor plus operator parseInt() native method This will be used to parse string to number in decimal places parseInt is an inbuilt method that takes input number in the form of a string and return number. Syntax The string is a numeric string enclosed in single or double-quotes. Radix is an optional parameter, possible values are 16 - hexa number, 8 - Octa number 2- Binary number 10 - Decimal number ie default The possible output is Valid number - if string is valid numeric values enclosed in single or double quote NAN - input string is either not valid number or null or undefined. console.log(parseInt('32', 10)) // outpus 32 as output console.log(parseInt('45')) // outpus 45 as output console.log(parseInt('45abc')) // outpus 45 as output console.log(parseInt('null')) // outpus NaN as output console.log(parseInt('45')) // outpus 45 as output parseFloat() method parseFloat is inbuilt method accepts a number in string format and convert to number in floating number format. The possible output is Valid floating number - if the string is valid numeric values enclosed in single or double quote NAN - input string is not valid numeric value or null or undefined. Syntax is parseFloat(string, radix) console.log(parseFloat('32.32')) // outpus 32.32 as output This method is useful for converting to floating numbers which contains decimal places Number() method Number() constructor takes a string as a parameter and returns number as output console.log(Number('12')) // outpus 12 as output console.log(Number('null')) // outpus NaN as output console.log(Number('undefined')) // outpus NaN as output plus unary operator This is a unary operator which accepts only one operand. This accepts strings, boolean values and all type of numbers and converts to numbers syntax Input is string boolean all number types - Hexa, octal, floating numbers The possible output is Valid number - if the string is valid numeric values enclosed in single or double quote NAN - input string is not valid numeric value or null or undefined. console.log(+'44') // outputs 44 console.log(+'44.23') // outputs 44.23 console.log(+true) // outputs 1 console.log(+false) // outputs 0 console.log(+undefined) // outputs 0 console.log(+null) // outputs 0 Conclusion It is very important to learn possible ways of typescript conversion with primitive types. Most of the above examples will work in javascript because typescript is a superset of javascript. I hope you learned many ways to get the string to/from the number in typescript. In addition, make sure you are sending valid string with numbers for conversion, In this way, It is a very easy way to parse to numbers. Reference

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