apply() JavaScript 函数

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#javascript #前端 #开发语言

本文介绍了JavaScript中的apply()和call()方法，它们用于改变函数调用时的上下文（this值）。apply()接收一个参数数组，而call()接受单独的参数。文中通过示例展示了如何在不同对象上调用方法，以及如何在没有内置max方法的数组上使用Math.max()。在严格模式和非严格模式下，apply()的第一个参数处理也有所不同。

使用 apply() 方法，您可以编写一个可用于不同对象的方法。

<!DOCTYPE html>
<html>
<body>

<h2>JavaScript Functions</h2>
<p>In this example the fulllName method of person is <b>applied</b> on person1:</p>

<p id="demo"></p>

<script>
const person = {
  fullName: function() {
    return this.firstName + " " + this.lastName;
  }
}

const person1 = {
  firstName:"John",
  lastName: "Doe"
}

document.getElementById("demo").innerHTML = person.fullName.apply(person1); 
</script>

</body>
</html>

call() 和 apply() 的区别

call() 方法单独接受参数。

apply() 方法将参数作为数组。

如果您想使用数组而不是参数列表，apply() 方法非常方便。

带参数的 apply() 方法

const person = {
fullName: function(city, country) {
    return this.firstName + " " + this.lastName + "," + city + "," + country;
  }
}

const person1 = {
  firstName:"John",
lastName: "Doe"
}

person.fullName.apply(person1, ["Oslo", "Norway"]);

<!DOCTYPE html>
<html>
<body>

<h2>JavaScript Functions</h2>
<p>In this example the fulllName method of person is <b>applied</b> on person1:</p>

<p id="demo"></p>

<script>
const person = {
  fullName: function(city, country) {
    return this.firstName + " " + this.lastName + "," + city + "," + country;
  }
}

const person1 = {
  firstName:"John",
  lastName: "Doe"
}

document.getElementById("demo").innerHTML = person.fullName.apply(person1, ["Oslo", "Norway"]); 
</script>

</body>
</html>

与 call() 方法相比：

const person = {
  fullName: function(city, country) {
    return this.firstName + " " + this.lastName + "," + city + "," + country;
  }
}

const person1 = {
firstName:"John",
lastName: "Doe"
}

person.fullName.call(person1, "Oslo", "Norway");
<!DOCTYPE html>
<html>
<body>

<h2>JavaScript Functions</h2>
<p>This example calls the fullName method of person, using it on person1:
</p>

<p id="demo"></p>

<script>
const person = {
  fullName: function(city, country) {
    return this.firstName + " " + this.lastName + "," + city + "," + country;
  }
}

const person1 = {
  firstName:"John",
  lastName: "Doe"
}

const person2 = {
  firstName:"Mary",
  lastName: "Doe"
}

document.getElementById("demo").innerHTML = person.fullName.call(person1, "Oslo", "Norway"); 
</script>

</body>
</html>

在数组上模拟 Max 方法

Math.max(1,2,3);

由于 JavaScript 数组没有 max() 方法，您可以改为应用 Math.max() 方法。

Math.max.apply(null, [1,2,3]); // Will also return 3
<!DOCTYPE html>
<html>
<body>

<h2>JavaScript apply()</h2>
<p>This example returns the highest number in an array of numbers:</p>

<p id="demo"></p>

<script>
document.getElementById("demo").innerHTML = Math.max.apply(null, [1,2,3]); 
</script>

</body>
</html>
第一个参数 (null) 无关紧要。本示例中未使用它。

这些示例将给出相同的结果：

Math.max.apply(Math, [1,2,3]); // Will also return 3
<!DOCTYPE html>
<html>
<body>

<h2>JavaScript apply()</h2>
<p>This example returns the highest number in an array of numbers:</p>

<p id="demo"></p>

<script>
document.getElementById("demo").innerHTML = Math.max.apply(Math, [1,2,3]); 
</script>

</body>
</html>
Math.max.apply(" ", [1,2,3]); // Will also return 3

Math.max.apply(0, [1,2,3]); // Will also return 3