Typescript if undefined

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Typescript if undefined

Go back to the homepage Published May 26, 2018, Last Updated May 29, 2019 A variable is said to be "undefined" if it has been declared but not initialized. Whereas "null" is assigned to a variable whose value is absent at that moment.In simple words, when we do not assign value to a variable, JavaScript engine treats it as undefined. The null value is assigned by programmer to indicate that variable has nothing inside of it, but intend to have value later in program execution.1. UndefinedSee below JavaScript statements to understand the undefined. var myVar; //Variable declaration without assigning any value to it console.log( myVar ); //undefined console.log( typeof(myVar) ); //undefined console.log( undeclaredVar ); //Uncaught ReferenceError: undeclaredVar is not defined If we see a variable with value in undefined, then we know that:The variable has been declared in the program.There is no value assignment done for the variable.If we check the type of variable, it will be undefined. Please note that is one of primitive types [string, number, boolean, null, undefined, symbol (new in ES6)] in JavaScript.If a function returns undefined, it didn't returned any value.undefined is not reserved keyword in JavaScript.2. Nullnull represents intentional absence of value. Conceptually, it's very much like null in other programming languages like Java, C# etc. null expresses a lack of identification, indicating that a variable points to no object. var myVar = null; //Variable declared and assigned to null console.log( myVar ); //null console.log( typeof(myVar) ); //object Please note that null is of type object with a valid value, no properties, is non-mutable, and only a single instance of the same exists in the system at all times.3. Using equality operators with null and undefinedAlways remember to use strict equality operator ( === ) for comparing variables with null and undefined, when we want to clearly distinguish between null and undefined variables.Read More: == vs === in JavaScript var myUndefinedVar; //undefined var myNullVar = null; //null myUndefinedVar == myNullVar; //true - which not correct as both are different myUndefinedVar === myNullVar; //false - correct behavior 4. Null evaluates to zeroOne major difference between null and undefined is in the way they are converted to primitive types.null is converted to zero (0) while performing primitive operations.undefined is converted to NaN. var myUndefinedVar; //undefined var myNullVar = null; //null myUndefinedVar + 100; //NaN myNullVar + 100; //100 Drop me your questions in comments section.Happy Learning !! TypeScript 3.7 added support for the ?? operator, which is known as the nullish coalescing operator. We can use this operator to provide a fallback value for a value that might be null or undefined. #Truthy and Falsy Values in JavaScript Before we dive into the ?? operator, let's recall that JavaScript values can either be truthy or falsy: when coerced to a Boolean, a value can either produce the value true or false. In JavaScript, the following values are considered to be falsy: false 0 -0 0n NaN "" null undefined All other JavaScript values will produce the value true when coerced to a Boolean and are thus considered truthy. #Providing Fallback Values with the ?? Operator The ?? operator can be used to provide a fallback value in case another value is null or undefined. It takes two operands and is written like this: value ?? fallbackValue If the left operand is null or undefined, the ?? expression evaluates to the right operand: null ?? "n/a" // "n/a" undefined ?? "n/a" // "n/a" Otherwise, the ?? expression evaluates to the left operand: false ?? true // false 0 ?? 100 // 0 "" ?? "n/a" // "" NaN ?? 0 // NaN Notice that all left operands above are falsy values. If we had used the || operator instead of the ?? operator, all of these expressions would've evaluated to their respective right operands: false || true // true 0 || 100 // 100 "" || "n/a" // "n/a" NaN || 0 // 0 This behavior is why you shouldn't use the || operator to provide a fallback value for a nullable value. For falsy values, the result might not be the one you wanted or expected. Consider this example: type Options = { prettyPrint?: boolean }; function serializeJSON(value: unknown, options: Options): string { const prettyPrint = options.prettyPrint ?? true; // ... } The expression options.prettyPrint ?? true lets us provide the default value true in case that the prettyPrint property contains the value null or undefined. If prettyPrint contains the value false, the expression false ?? true still evaluates to false, which is exactly the behavior we want here. Note that using the || operator here would lead to incorrect results. options.prettyPrint || true would evaluate to true for the values null and undefined, but also for the value false. This would clearly not be intended. I've seen this happen in practice a handful of times, so make sure to keep this case in mind and use towards the ?? operator instead. #Compiled Output: ES2020 and Newer The nullish coalescing operator has reached Stage 4 ("Finished") of the TC39 process and is now officially part of ES2020. Therefore, the TypeScript compiler will emit the ?? operator as is without any downleveling when you're targeting "ES2020" (or a newer language version) or "ESNext" in your tsconfig.json file: { "compilerOptions": { "strict": true, "target": "ES2020" } } So, this simple expression will be emitted unchanged: value ?? fallbackValue If you're planning on using the ?? operator while targeting "ES2020" or a newer language version, head over to and node.green and make sure that all the JavaScript engines you need to support have implemented the operator. #Compiled JavaScript Output: ES2019 and Older If you're targeting "ES2019" or an older language version in your tsconfig.json file, the TypeScript compiler will rewrite the nullish coalescing operator into a conditional expression. That way, we can start using the ?? operator in our code today and still have the compiled code successfully parse and execute in older JavaScript engines. Let's look at the same simple ?? expression again: value ?? fallbackValue Assuming we're targeting "ES2019" or a lower language version, the TypeScript compiler will emit the following JavaScript code: value !== null && value !== void 0 ? value : fallbackValue The value variable is compared against both null and undefined (the result of the expression void 0). If both comparisons produce the value false, the entire expression evaluates to value; otherwise, it evaluates to fallbackValue. Now, let's look at a slightly more complex example. Instead of a simple value variable, we're going to use a getValue() call expression as the left operand of the ?? operator: const value = getValue() ?? fallbackValue; In this case, the compiler will emit the following JavaScript code (aside from whitespace differences): var _a; const value = (_a = getValue()) !== null && _a !== void 0 ? _a : fallbackValue; You can see that the compiler generated an intermediate variable _a to store the return value of the getValue() call. The _a variable is then compared against null and void 0 and (potentially) used as the resulting value of the entire expression. This intermediate variable is necessary so that the getValue function is only called once. #Compiled Output: Checking for null and undefined You might be wondering why the compiler emits the following expression to check the value variable against null and undefined: value !== null && value !== void 0 Couldn't the compiler emit the following shorter check instead? value != null Unfortunately, it can't do that without sacrificing correctness. For almost all values in JavaScript, the comparison value == null is equivalent to value === null || value === undefined. For those values, the negation value != null is equivalent to value !== null && value !== undefined. However, there is one value for which these two checks aren't equivalent, and that value is document.all: document.all === null // false document.all === undefined // false document.all == null // true document.all == undefined // true The value document.all is not considered to be strictly equal to either null or undefined, but it is considered to be loosely equal to both null and undefined. Because of this anomaly, the TypeScript compiler can't emit value != null as a check because it would produce incorrect results for document.all. You can read more about this curious behavior in an answer to the Why is document.all falsy? question on Stack Overflow. Oh, the things we do for web compatibility. NEWBEDEVPythonJavascriptLinuxCheat sheet Late to the story but I think some details are overlooked? if you use if (uemail !== undefined) { //some function } You are, technically, comparing variable uemail with variable undefined and, as the latter is not instantiated, it will give both type and value of 'undefined' purely by default, hence the comparison returns true. But it overlooks the potential that a variable by the name of undefined may actually exist -however unlikely- and would therefore then not be of type undefined. In that case, the comparison will return false. To be correct one would have to declare a constant of type undefined for example: const _undefined: undefined and then test by: if (uemail === _undefined) { //some function } This test will return true as uemail now equals both value & type of _undefined as _undefined is now properly declared to be of type undefined. Another way would be if (typeof(uemail) === 'undefined') { //some function } In which case the boolean return is based on comparing the two strings on either end of the comparison. This is, from a technical point of view, NOT testing for undefined, although it achieves the same result. In TypeScript, you assign types to variables and function parameters to catch type related errors early, even before you run the code. But TypeScript doesn't warn you when you assign null and undefined to the variables of common types. For instance, take the following example. function appendDomain(url: string) { return url.concat('.com'); } console.log(appendDomain('amitmerchant')); I've written this simple function in TypeScript that accepts its only parameter url which is of type string. Now, this will work fine if you compile it down to the JavaScript file providing I have the following tsconfig.json file setup. { "compilerOptions": { "target": "esnext" } } Now, if I change provide null instead of a valid string, compile and run the generated JS code, I would get the following error. That's because we passed an argument (null) for which JavaScript can not find concat method on it as it's not a valid string. But TypeScript didn't gave any type error because in TypeScript, null and undefined can be assigned to any type. So, the following is also perfectly fine as per TypeScript's type checker. let title: string; title = 'Foo Bar'; title = null; This can cause some serious issues if not handled properly. This behavior can be fixed by adding a compiler option called strictNullChecks and set it to true in tsconfig.json like so. { "compilerOptions": { "target": "esnext", "strictNullChecks": true } } Upon adding this option, TypeScript's compiler will start showing typing errors because null and undefined are no longer valid values for any type. So, for the above example, it will show the following error. Type 'null' is not assignable to type 'string'. To fix our appendDomain function, we can add more types to the arguments to include null and undefined like so and checking if the provided value in the function is of type string explicitly like so. function appendDomain(url: string | null | undefined) { if (typeof url === 'string') { return url.concat('.com'); } return url; } console.log(appendDomain('amitmerchant')); console.log(appendDomain(null)); console.log(appendDomain(undefined)); The function can now gracefully accepts null and undefined and returns values accordingly. Using a juggling-check, you can test both null and undefined in one hit: if (x == null) { If you use a strict-check, it will only be true for values set to null and won't evaluate as true for undefined variables: if (x === null) { You can try this with various values using this example: var a: number; var b: number = null; function check(x, name) { if (x == null) { console.log(name + ' == null'); } if (x === null) { console.log(name + ' === null'); } if (typeof x === 'undefined') { console.log(name + ' is undefined'); } } check(a, 'a'); check(b, 'b'); Output "a == null" "a is undefined" "b == null" "b === null" My go-to has always been the ternary operator for assigning a value to a variable conditionally. But ever since I discovered that "||" can be used as a selector operator, I've been using that more. I find my code so much easier to read Yes, it takes some time to wrap your head around it. But once you grasp the concept, it's super handy. Now I don't think less code makes your code better. But in this instance, I prefer the || operator # Understanding the || Operator I'm sure most of you thought the || is only used for boolean checks like this: BUT! You can also use it to evaluate the selected expression and produce a value. And that's why it's helpful to think of the logical operator as also a selector operator. When it's used with non-boolean values, the || operator will return a non-boolean value of one of the specified expression or operands. Head explosion yet?! No worries, let me explain it the way Kyle Simpson explains it. He is the author of "You Don't Know JavaScript " - a free JavaScript e-book. The value produced by a && or || operator is not necessarily of type Boolean. The value produced will always be the value of one of the two operand expressions. Alright, let's look at an example. As long as the 1st expression (left side) is truthy, it will always be the one selected. However, if the 1st expression (left side) is ever falsy, then the 2nd expression (right side) will be by default output. And that's why this || is known as the operator to set default values. # Using Default Value as Function Parameter Quite often you would see || being used like this: Note: this is not the recommended way anymore. It's way better to ES6's default parameters. Because quite often, you might not want the default to kick in for ALL falsy values -- I'll explain falsy values in the next section. Most likely, we only want the default value to be set if no value or undefined is passed as the argument. The better solution with ES6 Default Parameters # Falsy Values In the || operator, the 2nd expression (right side) is only evaluated IF the 1st expression (left side). So let's check what constitutes a falsy value. (I wrote another blog post on Falsy Values, which you can read it here ) # Compared to the && operator In my previous post, I wrote about the && operator. (Read it here ). The && is also known as the Guard Operator. So here's a quick summary of the difference: ||: 1st expression is always outputted. The 2nd expression only gets outputted if the 1st expression is falsy. &&: 1st expression is outputted if it's FALSY. The 2nd expression only get outputted if the 1st expression is truthy. # What is the Elvis Operator This is an interesting one. In JavaScript we have || to set default values. In other languages such as C++, this behavior is similar to the Elvis Operator which is denoted as ?:. As to why it's called the Elvis Operator: Image Credit to # When to use which? Now that you understand Falsy Values, let's figure out which way of the 3 ways is better to use. Logical Operator || This is great if you want to capture all falsy values. It's less code and it's way easier to read. Assuming that everyone understands all 3 behaviors, of course. NOTE: I'm not saying less code is always better, one can easily try to be too clever and shorten the code which renders it unreadable. We write code for others, it's a form of communication. It's always better to go with the option that conveys understanding over being clever. Ternary Operator Let's say we don't want to capture ALL falsy values. And we only want the default value to kick in when it's undefined If/Else This is the option with the MOST control. And it's something I would absolutely go for if say, I need to perform an additional action. # Resources

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