JavaScript determines variable data type

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';
}

But in some cases it won’t work, for example:

var a;
alert(isNumber(parseInt(a)));

What pops up here is true, as shown in the figure below:

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);
}

After modification, false will pop up, as shown in the figure below:

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";
}

Test code:

Run result:

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";
}

Test code:

Running results:

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";
}

Test code:

Run Result:

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';
}

Test code:

Running result:

6. Null value (Null)

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

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

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

Running result:

##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;
}

## Test code:

Run results:

The 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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