Take you to understand JavaScript variable types and conversion between variables

This article brings you relevant knowledge about javascript. It mainly introduces JavaScript variable types and conversions between variables in detail. The sample code in the article is introduced in great detail and has certain Reference value, I hope it will be helpful to everyone.

[Related recommendations: javascript video tutorial, web front-end】

1. Variable type

<script>
        var num =10;
        console.log(typeof num);
</script>

• typeof: Declaration of variable type

• ##alert: Pop-up alert box

• prompt: Input and output statements

1.1 Color of variable type

Identify variable type according to the color of the console ：

• Purple: Numeric type

• Black: Character type

• Blue: Boolean type (

  true false)

• Light gray:

  undefined null

2. Literal

Literal is the representation of a fixed value in the source code, that is, the literal represents how to express this value.

3. Data type conversion

3.1. Convert to string type

1. Add sign to concatenate string

console.log(num 'I am a string')

2.toString() Convert to a string

The string representation of a number. For example, when radix is ​​2, the NumberObject is converted to a string representing the binary value.

The syntax is:

number.toString(radix)

Parameter Description

radix Optional. Specifies the base of the number, which is an integer between 2 and 36. If this parameter is omitted, base 10 is used. Note, however, that the ECMAScript standard allows implementations to return any value if the parameter is a value other than 10.

• 2 - The number is displayed as a binary value

• 8 - The number is displayed as an octal value

• 16 - The number is displayed as a hexadecimal value

Example is as follows:

var num=10;
var str=num.toString();
console.log(str);

3.string (variable)

3.2 Convert to numeric type (key point)

parseInt(string) function - integer numerical type

parseInt() function can parse a character String and returns an integer.

When the value of parameter radix is ​​0, or the parameter is not set, parseInt() will determine the base of the number based on string.

When the parameter radix is ​​omitted, JavaScript defaults to the radix of numbers as follows:

If string starts with "0x", parseInt() will parse the rest of the string into hexadecimal integers.

If string begins with 0, then ECMAScript v3 allows an implementation of parseInt() to parse subsequent characters into octal or hexadecimal digits.

If string starts with a number from 1 to 9, parseInt() will parse it into a decimal integer.

Grammar

parseInt(string, radix)

The example is as follows

var age=prompt('请输入您的年龄');
        console .log(parseInt(age));
        console.log(parseInt('3.14'));  //取整 3
        console.log(parseInt('3.98'));  //取整 3
        console.log(parseInt('120px'));  //取整 120

2.parseFloat(string) function--floating point numerical type

parseFloat( ) function parses a string and returns a floating point number.

This function specifies whether the first character in the string is a number. If it is, the string is parsed until it reaches the end of the number, and the number is returned as a number rather than as a string.

Grammar

parseFloat(string)

The example is as follows

console.log(parseFloat('3.14'));
console.log(parseFloat('120px')); //120会去掉px单位

3.Use number (variable)

var str='123';
console.log(number(str);
console.log(number('123'));

4.Use Arithmetic operations - * / Implicit conversion

console.log('12'-0); //12
console.log('123'-'120'); //3
console.log('123'*1); //123

3.3 Convert to Boolean

Boolean() function

Boolean object represents two Value: "true" or "false"

The following code defines a Boolean object named myBoolean:

var myBoolean=new Boolean();

If the Boolean object has no initial value or its value is:

• 0

• -0

• null

• ""

• false

• undefined

• NaN

Then the value of the object is false. Otherwise, its value is true (even when the variable value is the string "false")!

console.log(boolean(''));//false
console.log(boolean(0));//false
console.log(boolean(NaN));//false
console.log(boolean(null));//false
console.log(boolean(undefined));//false

The rest are true.

【Related recommendations:

javascript video tutorial, web front-end】

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