In JavaScript, there are many ways to determine the data type. Here are some common methods:
Use the typeof operator: typeof is a unary operator in JavaScript that returns the data type of a value. Example:
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console.log(typeof "hello"); // Output: "string"
console.log(typeof 42); // Output: "number"
console.log(typeof true); // Output: "boolean "
console.log(typeof undefined); // output: "undefined"
console.log(typeof null); // output: "object" (this is a legacy issue)
console.log(typeof [1, 2, 3 ]); // Output: "object"
console.log(typeof { name: "John", age: 30 }); // Output: "object"
console.log(typeof function () {}); // Output : "function"
uses the instanceof operator: The instanceof operator is used to check whether an object is an instance of a particular type. Example:
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console.log("hello" instanceof String); // output:
console.log(42 instanceof Number); // Output: false
console.log(new Number(42) instanceof Number); // Output: true
console.log(true instanceof Boolean); // Output: false
console.log( new Boolean(true) instanceof Boolean); // Output: true
console.log([1, 2, 3] instanceof Array); // Output: true
console.log({ name: "John", age: 30 } instanceof Object); // Output: true
console.log(function () {} instanceof Function); // Output: true It
should be noted that the instanceof operator can only be used to check the type of the object, for basic data types (such as characters string, number, boolean) is invalid.
Use the Object.prototype.toString method: This is a generic method that returns the concrete type of a value. Example:
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console.log(Object.prototype.toString.call("hello")); // output: "[object String]"
console.log(Object.prototype.toString.call(42)); // Output: "[object Number]"
console.log(Object.prototype.toString.call(true)); // Output: "[object Boolean]"
console.log(Object.prototype.toString.call(undefined)); // Output: "[object Undefined]"
console.log(Object.prototype.toString.call(null)); // Output: "[object Null]"
console.log(Object.prototype.toString.call([1 , 2, 3])); // Output: "[object Array]"
console.log(Object.prototype.toString.call({ name: "John", age: 30 })); // Output: "[ object Object]"
console.log(Object.prototype.toString.call(function () {})); // Output: "[object Function]"
Using the Array.isArray method: Array.isArray is a static method that checks whether a value for the array. Example:
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console.log(Array.isArray([1, 2, 3])); // output: true
console.log(Array.isArray({ name: "John", age: 30 })); / / Output: false
console.log(Array.isArray("hello")); // Output: false
Use the combination of typeof and constructor: Sometimes using the typeof operator may get inaccurate results, which can be combined with the constructor attribute judge. Example:
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console.log(typeof "hello"); // Output: "string"
console.log("hello".constructor === String); // Output: true
console.log(typeof 42); // Output: "number"
console.log((42).constructor === Number); // Output: true
console.log(typeof true); // Output: "boolean"
console.log((true).constructor === Boolean); // Output: true
console.log(typeof undefined); // output: "undefined"
console.log((undefined).constructor === undefined); // output: false
console.log(typeof null); // Output: "object"
console.log((null).constructor === null); // Output: false
console.log(typeof [1, 2, 3]); // Output: "object"
console.log([1, 2, 3].constructor === Array); // Output: true
console.log(typeof { name: "John", age: 30 }); // 输出: "object"
console.log({ name: "John", age: 30 }.constructor === Object); // 输出: true
console.log(typeof function () {}); // output: "function"
console.log((function () {}).constructor === Function); // output: true
Note that the above method is not Completely reliable, because there are some special cases and boundary conditions for data type judgment in JavaScript. When judging the data type, the appropriate method should be selected according to the actual needs, and sufficient testing and verification should be carried out in actual use.