JavaScript type conversion
Number() converts to a number, String() converts to a string, and Boolean() converts to a Boolean value.
JavaScript Data Types
There are 5 different data types in JavaScript:
string
number
boolean
- ##object
- function
- Object
- Date
- Array
- null
- undefined
typeof operatorYou can use the
typeof operator to view the data type of a JavaScript variable.
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<html>
<head>
<meta charset="utf-8">
<title>php.cn</title>
</head>
<body>
<p> typeof 操作符返回变量、对象、函数、表达式的类型。</p>
<p id="demo"></p>
<script>
document.getElementById("demo").innerHTML =
typeof "john" + "<br>" +
typeof 3.14 + "<br>" +
typeof NaN + "<br>" +
typeof false + "<br>" +
typeof [1,2,3,4] + "<br>" +
typeof {name:'john', age:34} + "<br>" +
typeof new Date() + "<br>" +
typeof function () {} + "<br>" +
typeof myCar + "<br>" +
typeof null;
</script>
</body>
</html>Run instance»Click the "Run instance" button to view the online instance
- The data type of NaN is number
- The data type of array (Array) is object
- The data type of date (Date) is object
- The data type of null is object
- It is an undefined variable The data type is undefined
typeof, because they all return Object.
constructor property
constructor property returns the constructor function for all JavaScript variables.
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<html>
<head>
<meta charset="utf-8">
<title>php.cn</title>
</head>
<body>
<p> constructor 属性返回变量或对象的构造函数。</p>
<p id="demo"></p>
<script>
document.getElementById("demo").innerHTML =
"john".constructor + "<br>" +
(3.14).constructor + "<br>" +
false.constructor + "<br>" +
[1,2,3,4].constructor + "<br>" +
{name:'john', age:34}.constructor + "<br>" +
new Date().constructor + "<br>" +
function () {}.constructor;
</script>
</body>
</html>Run instance»Click the "Run instance" button to view the online instance
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>php.cn</title>
</head>
<body>
<p>判断是否为数组。</p>
<p id="demo"></p>
<script>
var fruits = ["Banana", "Orange", "Apple", "Mango"];
document.getElementById("demo").innerHTML = isArray(fruits);
function isArray(myArray) {
return myArray.constructor.toString().indexOf("Array") > -1;
}
</script>
</body>
</html>Run Instance»Click the "Run Instance" button View online examples
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>php.cn</title>
</head>
<body>
<p>判断是否为日期。</p>
<p id="demo"></p>
<script>
var myDate = new Date();
document.getElementById("demo").innerHTML = isDate(myDate);
function isDate(myDate) {
return myDate.constructor.toString().indexOf("Date") > -1;
}
</script>
</body>
</html>Run Instance»Click the "Run Instance" button View online examples
JavaScript Type Conversion
JavaScript variables can be converted to new variables or other data types:
By using JavaScript functions
Automatic conversion through JavaScript itself
Convert numbers to strings
Global methodString() You can Convert numbers to strings.
This method can be used for any type of numbers, letters, variables, expressions:
Example
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<html>
<head>
<meta charset="utf-8">
<title>php.cn</title>
</head>
<body>
<p> String() 方法可以将数字转换为字符串。</p>
<p id="demo"></p>
<script>
var x = 123;
document.getElementById("demo").innerHTML =
String(x) + "<br>" +
String(123) + "<br>" +
String(100 + 23);
</script>
</body>
</html>Running example »
Click the "Run Instance" button to view the online instance
Number method toString() also has the same effect.
Instance
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>php.cn</title>
</head>
<body>
<p>toString() 方法将数字转换为字符串。</p>
<p id="demo"></p>
<script>
var x = 123;
document.getElementById("demo").innerHTML =
x.toString() + "<br>" +
(123).toString() + "<br>" +
(100 + 23).toString();
</script>
</body>
</html>Run instance»
Click the "Run instance" button to view the online instance
In the Number method chapter, you can find more methods for converting numbers to strings:
| Method | Description |
|---|---|
| toExponential() | Convert the value of the object to exponential notation. |
| toFixed() | Convert the number to a string, and the result will have the specified number of digits after the decimal point. |
| toPrecision() | Format the number to the specified length. |
Convert Boolean value to string
Global method String() You can convert Boolean value to string.
Boolean methodtoString() also has the same effect.
// Return "false"true.toString()
// Return "true"Global method
String()
String(Date())
// Return Thu Jul 17 2014 15:38:19 GMT+0200 (W. Europe Daylight Time)
Date().toString() // Return Thu Jul 17 2014 15:38:19 GMT+0200 (W. Europe Daylight Time)
| getDate () | Returns the day of the month (1 ~ 31) from the Date object. | ||||
|---|---|---|---|---|---|
| getDay() | Returns the day of the week (0 ~ 6) from the Date object. | ||||
| getFullYear() | Returns the year as a four-digit number from a Date object. | ||||
| getHours() | Returns the hours (0 ~ 23) of the Date object. | ||||
| getMilliseconds() | Returns the milliseconds (0 ~ 999) of the Date object. | ||||
| getMinutes() | Returns the minutes (0 ~ 59) of the Date object. | ||||
| getMonth() | Returns the month (0 ~ 11) from the Date object. | ||||
| getSeconds() | Returns the number of seconds in the Date object (0 ~ 59). | ||||
| getTime() | Returns the number of milliseconds since January 1, 1970. | ||||
Convert a string to a numberGlobal method Number() You can convert a string to a number. Strings containing numbers (such as "3.14") are converted to numbers (such as 3.14). Empty strings are converted to 0. Other strings will be converted to NaN (not a number). Number("3.14") // Return 3.14 Number(" ") // Return 0 Number("") Returns NaN In the Number method chapter, you can see more methods for converting strings to numbers:
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