How to determine if a value is a number in javascript

In JavaScript, we can use a series of functions and methods to determine whether a value is a number. The following are some commonly used methods and examples:

1. Use the typeof operator to determine the type of a value

The typeof operator in JavaScript can return the type of a value, and the types include "string", "number", "boolean", "object", "function", "undefined" and "symbol". When the value is a number, the typeof operator returns "number".

Sample code:

console.log(typeof 42); // 输出 "number"
console.log(typeof "42"); // 输出 "string"

2. Use the isNaN() function to determine whether it is a non-number

The isNaN() function in JavaScript is used to determine a value Whether it is not a number (NaN). The isNaN() function returns true when the value is not a number, otherwise it returns false.

Sample code:

console.log(isNaN(42)); // 输出 false
console.log(isNaN("42")); // 输出 false
console.log(isNaN("hello")); // 输出 true

It should be noted that when the incoming parameter cannot be converted to a number, the isNaN() function will also return true. Therefore, when using this function to determine whether a value is a number, you need to convert it to a numeric type first.

3. Use the Number() function to convert a string into a number

The Number() function in JavaScript can convert a string into a number. If the string can be converted to a legal number, the converted numeric value is returned; otherwise, NaN is returned.

Sample code:

console.log(Number("42")); // 输出 42
console.log(Number("hello")); // 输出 NaN

4. Use regular expressions to determine whether it is a number

Regular expressions in JavaScript can be used to match numbers. The following are regular expressions for integers and floating point numbers:

// 判断是否为整数
/^-?\d+$/

// 判断是否为浮点数
/^-?\d+\.\d+$/

Sample code:

console.log(/^-?\d+$/.test(42)); // 输出 true
console.log(/^-?\d+$/.test("42")); // 输出 true
console.log(/^-?\d+$/.test("42.5")); // 输出 false

console.log(/^-?\d+\.\d+$/.test(3.14)); // 输出 true
console.log(/^-?\d+\.\d+$/.test("3.14")); // 输出 true
console.log(/^-?\d+\.\d+$/.test("3.14.5")); // 输出 false

Summary:

The above are several commonly used methods to determine whether it is a number. In actual development, it is necessary to choose the appropriate method according to specific needs. Especially when using the typeof and isNaN() functions, you need to pay attention to some of their special cases to avoid unexpected results.

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