Different functions of question marks in javascript

JavaScript is a high-level programming language that is widely used in the development of web applications. In JavaScript, the question mark (?) has many different uses, and this article will explore these different uses in depth.

1. Ternary operator

In JavaScript, the question mark is used as part of the ternary operator, and its general syntax is as follows:

condition ? exprIfTrue : exprIfFalse

Where condition is a Boolean expression that returns exprIfTrue if it evaluates to true, otherwise exprIfFalse is returned.

For example, the following code uses the ternary operator to compare the size of two numbers:

 let x = 10;
 let y = 20;
 let result = (x > y) ? "x is greater than y" : "y is greater than x";
 console.log(result); // Output: "y is greater than x"

2. Query string

Question mark in URL also plays a very important role. In URLs, the question mark is usually used as the delimiter of the query string. The query string refers to the parameter part of the URL. It is a collection of key-value pairs separated by the "&" symbol.

For example, the following URL includes the query string "?q=javascript lessons", where q is the key and javascript lessons is the corresponding value.

https://www.google.com/search?q=javascript+lessons

In JavaScript, you can use the window.location.search property to get the query string contained after the question mark, for example:

 let queryString = window.location.search;
 console.log(queryString); // Output: "?q=javascript+lessons"

The query string is passed URL parameters are one of the standard ways of passing information. We can add parameters to the URL to pass parameters between browsers. This is very common in web development, especially when working with forms and using AJAX.

3. Optional function parameters

In JavaScript, you can use question marks to specify optional function parameters. In this case, if the parameter is not passed, its value is undefined.

For example, the following code defines a function to calculate the square value of a number:

function square(num, root = false) {
  if (root) {
    return Math.sqrt(num);
  } else {
    return num * num;
  }
}

This function has two parameters: num is to be calculated Number, root indicates whether to calculate the square root of this number. If the root parameter is not passed, it defaults to false. If you need to calculate square roots, set the root parameter to true:

console.log(square(4)); // Output: 16
console.log(square(4, true)); // Output: 2

4. Regular Expression

in JavaScript , question marks can also be used in regular expressions. Regular expressions are a pattern matching tool that finds specific text patterns in strings.

In regular expressions, the question mark is generally used as one of the metacharacters with special meaning. For example, a question mark can be used to indicate that the preceding character is optional. Here are some common examples of using question marks in regular expressions:

• Remove leading zeros (if any): str.replace(/^0 /, '')
• Make the previous match non-greedy (i.e. as short as possible): /a ?/
• Match optional singular or plural: /apple (s)?/

5. Type assertion

In TypeScript and Angular, the question mark can also be used as a type assertion operator. This is a special syntax used to specify the type of a variable. In this case, a question mark is placed after the type name, indicating that the value can be undefined or that type.

For example, suppose we have a variable message and we declare it as a string or undefined:

let message: string | undefined;

We can use question marks to convert message The variable is asserted to be of type string:

console.log(message?.toUpperCase());

In this case, if the value of message is undefined or is another type that is not a string, # is returned ##undefined.

Summary

In JavaScript, question marks have many different functions. It can be used as a ternary operator, a marker in URL query strings, an optional argument to a function, a metacharacter in regular expressions, and a type assertion operator. It's important to understand these different usages because they are all essential in web development.

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