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JavaScript determines variable data type

高洛峰
Release: 2017-01-14 10:12:23
Original
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I recently worked on a project, which involved the processing of js judgment of data types. I searched for relevant information on the Internet, and personally tested the judgment of various data types. It is absolutely safe. Below, the editor will share with you a summary of the specific content for your reference!

1. Data types in JS

1. Numeric type (Number): including integers and floating point numbers.

2. Boolean

3. String

4. Object

5. Array

6. Null

7. Undefined

2. Determine the data type of a variable

1. Numeric type (number)

The more commonly used judgment method is:

function isNumber(val){
return typeof val === 'number';
}
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But in some cases it won’t work, for example:

var a;
alert(isNumber(parseInt(a)));
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What pops up here is true, as shown in the figure below:

JavaScript determines variable data type

But in fact the variable a is NaN, which cannot be used for numerical operations.

So the above function can be modified as:

function isNumber(val){
return typeof val === 'number' && isFinite(val);
}
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After modification, false will pop up, as shown in the figure below:

JavaScript determines variable data type

By the way, let me introduce the JavaScript isFinite() function. The isFinite() function is used to check whether its parameter is infinity. If number is a finite number (or can be converted to a finite number), then it returns true. Otherwise, if number is NaN (not a number), or a positive or negative infinity number, false is returned.

2. Boolean

The judgment of Boolean type is relatively simple and can be judged by the following method:

/*
判断变量val是不是布尔类型
*/
function isBooleanType(val) {
return typeof val ==="boolean";
}
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Test code:

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Run result:

JavaScript determines variable data type

3. String (String)

The judgment of string type is relatively simple, you can Use the following method to judge:

/*
判断变量是不是字符串类型
*/
function isStringType(val) {
return typeof val === "string";
}
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Test code:

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Running results:

JavaScript determines variable data type

4. Undefined

The undefined judgment is relatively simple and can be judged by the following method:

/*
判断变量是不是Undefined
*/
function isUndefined(val) {
return typeof val === "undefined";
}
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Test code:

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Run Result:

JavaScript determines variable data type

5. Object (Object)

Since typeof will also return object when the variable is Null, Object cannot Use typeof to judge directly.

It should look like this:

function isObj(str){
if(str === null || typeof str === 'undefined'){
return false;
}
return typeof str === 'object';
}
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Test code:

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Running result:

JavaScript determines variable data type

6. Null value (Null)

To judge the null value, use val === null to test

function isNull(val){
return val === null;
}
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Code:

/*
判断变量是不是null
*/
function isNull(val){
return val === null;
}
/*测试变量*/
var a;
var b = null;
var c = "str";
//弹出运行结果
alert("变量a是null的判断结果是:"+isNull(a));//false
alert("变量b是null类型的判断结果是:"+isNull(b));//true
alert("变量c是null类型的判断结果是:"+isNull(c));//false
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Running result:

JavaScript determines variable data type

##7. Array


The array type cannot be determined by typeof. Because when the variable is an array type, typeof will return object.


There are two methods to determine the array type:

/*判断变量arr是不是数组
方法一
*/
function isArray(arr) {
return Object.prototype.toString.apply(arr) === '[object Array]';
}
/*判断变量arr是不是数组
方法二
*/
function isArray(arr) {
if(arr === null || typeof arr === 'undefined'){
return false;
}
return arr.constructor === Array;
}
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## Test code:

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Run results:

JavaScript determines variable data typeThe above content is a summary of JavaScript knowledge points introduced by the editor (six): JavaScript determines variable data type Related knowledge, I hope it will be helpful to everyone. If you want to know more, please pay attention to the PHP Chinese website!

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