JavaScript function parameters

The JavaScript function does not perform any check on the value of the parameter.

Function explicit parameters (Parameters) and implicit parameters (Arguments)

In the previous tutorial, we have learned about the explicit parameters of the function:

functionName(parameter1, parameter2, parameter3) {
//The code to be executed...
}

Explicit parameters of functions are listed when the function is defined.

Implicit function parameters are the real values passed to the function when the function is called.

Parameter rules

When a JavaScript function is defined, the display parameter does not specify a data type.

JavaScript functions do not perform type detection on implicit parameters.

The JavaScript function does not detect the number of implicit parameters.

Default parameters

If no implicit parameters are provided when the function is called, the parameters will be set to: undefined

Sometimes this is Acceptable, but it is recommended to set a default value for the parameter:

Instance

<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>php中文网(php.cn)</title>
</head>
<body>

<p>设置参数的默认值。</p>
<p id="demo"></p>
<script>
function myFunction(x, y) {
    if (y === undefined) {
        y = 0;
    }    
    return x * y;
}
document.getElementById("demo").innerHTML = myFunction(4);
</script>

</body>
</html>

Run instance»

Click the "Run Instance" button to view the online instance

Or, easier way:

Instance

<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>php中文网(php.cn)</title>
</head>
<body>

<p>设置函数参数默认值。</p>
<p id="demo"></p>
<script>
function myFunction(x, y) {
    y = y || 0;
    return x * y;
}
document.getElementById("demo").innerHTML = myFunction(4);
</script>

</body>
</html>

Run instance»

Click the "Run instance" button to view the online instance

##If y has been defined , y || returns y, because y is true, otherwise returns 0, because undefined is false.

If too many parameters are set when the function is called, the parameters will not be referenced because the corresponding parameter names cannot be found. Can only be called using the arguments object.

Arguments object

JavaScript function has a built-in object arguments object.

The argument object contains the parameter array of the function call.

In this way you can easily find the value of the last parameter:

Example

<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>php中文网(php.cn)</title>
</head>
<body>

<p>查找最大的数。</p>
<p id="demo"></p>
<script>
function findMax() {
    var i, max = 0;
    for(i = 0; i < arguments.length; i++) {
		if (arguments[i] > max) {
            max = arguments[i];
        }
    }
    return max;
} 
document.getElementById("demo").innerHTML = findMax(4, 5, 6);
</script>

</body>
</html>

Running example »

Click the "Run Example" button to view the online example

Or create a function to count the sum of all values:

Example

<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>php中文网(php.cn)</title>
</head>
<body>

<p>计算所有参数之和：</p>
<p id="demo"></p>
<script>
function sumAll() {
    var i, sum = 0;
    for(i = 0; i < arguments.length; i++) {
        sum += arguments[i];
    }
    return sum;
} 
document.getElementById("demo").innerHTML =
	sumAll(1, 123, 500, 115, 44, 88);
</script>

</body>
</html>

Run Instance»

Click the "Run Instance" button to view the online instance