How does JavaScript determine data type? 8 ways to share

How does JavaScript determine the data type? This article will share with you 8 ways to use JS to determine data type, which can effectively help work and interviews. The interviewer smiled slightly after reading it.

1. typeof

• can only recognize basic types and reference types

Note:null,NaN,document.allJudgment

console.log(typeof null); // object console.log(typeof NaN); // number console.log(typeof document.all); // undefined

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2, constructor

• constuctorPoints to the constructor that created the instance object

Note thatnullandundefineddo not haveconstructor, andconstructorCan be rewritten

String.prototype.constructor = function fn() { return {}; }; console.log("云牧".constructor); // [Function: fn]

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3, instanceof

• Syntax:obj instanceof Type
• Function: JudgmentWhether objis an instance of theTypeclass, it can only be used to determine whether the reference data
• is implemented: whether the prototype object ofTypeisobjAn object on the prototype chain
• Note: The right operand must be a function or class

Handwritteninstanceof:

function myInstanceof(Fn, obj) { // 获取该函数显示原型 const prototype = Fn.prototype; // 获取obj的隐式原型 let proto = obj.__proto__; // 遍历原型链 while (proto) { // 检测原型是否相等 if (proto === prototype) { return true; } // 如果不等于则继续往深处查找 proto = proto.__proto__; } return false; }

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4. Is isPrototypeof

• on the prototype chain of the instance object?
• is basically equivalent toinstanceof

console.log(Object.isPrototypeOf({})); // false console.log(Object.prototype.isPrototypeOf({})); // true 期望左操作数是一个原型，{} 原型链能找到 Object.prototype

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5. Object.prototype.toString

• Use the feature of function dynamic this

function typeOf(data) { return Object.prototype.toString.call(data).slice(8, -1); } // 测试 console.log(typeOf(1)); // Number console.log(typeOf("1")); // String console.log(typeOf(true)); // Boolean console.log(typeOf(null)); // Null console.log(typeOf(undefined)); // Undefined console.log(typeOf(Symbol(1))); // Symbol console.log(typeOf({})); // Object console.log(typeOf([])); // Array console.log(typeOf(function () {})); // Function console.log(typeOf(new Date())); // Date console.log(typeOf(new RegExp())); // RegExp

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6. Duck type detection

• Check the type of its own attributes or the type of the execution result
• Usually used as a candidate solution
• Example:kindofandp-is-promise

p-is-promise：

const isObject = value => value !== null && (typeof value === "object" || typeof value === "function"); export default function isPromise(value) { return ( value instanceof Promise || (isObject(value) && typeof value.then === "function" && typeof value.catch === "function") ); }

kindof：

function kindof(obj) { var type; if (obj === undefined) return "undefined"; if (obj === null) return "null"; switch ((type = typeof obj)) { case "object": switch (Object.prototype.toString.call(obj)) { case "[object RegExp]": return "regexp"; case "[object Date]": return "date"; case "[object Array]": return "array"; } default: return type; } }

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7、Symbol.toStringTag

• Principle:Object.prototype.toStringwill read the value
• Applicable scenarios: custom type required
• Notes: Compatibility

class MyArray { get [Symbol.toStringTag]() { return "MyArray"; } } const arr = new MyArray(); console.log(Object.prototype.toString.call(arr)); // [object MyArray]

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8. Waiting comparison

• Principle: Compare with a fixed value
• Applicable scenarios:undefined,window,document,null, etc.

underscore.js:

##Summary

Method Basic data type Reference type Notes typeof √ × NaN、object、document.all ##constructor instanceof isPrototypeof toString Duck typing Symbol.toString Tag Etc. Compare Additional food: ES6 enhanced NaN

√ Part	√	can be rewritten
×	√	Multi-window, right constructor or class
×	√	Be careful with null and undefined
√	√	Be careful with built-in prototypes
-	√	Compatible as a last resort
×	√	Identify custom objects
√	√	Special objects

NaN and Number.NaN features

• typeof

  is followed by a number

I am not equal to myself
• delete

  cannot be deleted

isNaN

If it is not a number, if the incoming result of implicit conversion is

NaN
• , it will returntrue, otherwise it will returnfalse

  console.log(isNaN(NaN)); // true console.log(isNaN({})); // true

Number.isNaN

Determine whether a value is a number and whether the value is equal to

NaN

• console.log(Number.isNaN(NaN)); // true console.log(Number.isNaN({})); // false

Other judgments Whether

NaN

's methods

function isNaNVal(val) { return Object.is(val, NaN); } function isNaNVal(val) { return val !== val; } function isNaNVal(val) { return typeof val === "number" && isNaN(val); } // 综合垫片 if (!("isNaN" in Number)) { Number.isNaN = function (val) { return typeof val === "number" && isNaN(val); }; }

indexOf and includes

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indexOf
• are not searchableNaN,includesthen you can

  const arr = [NaN]; console.log(arr.indexOf(NaN)); // -1 console.log(arr.includes(NaN)); // true

[Recommended learning:

javascript advanced tutorial

】

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