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書名： Object-Oriented JavaScript（Second Edition）
作者名： Stoyan Stefanov Kumar Chetan Sharma
本章字數： 392字
更新時間： 2021-08-13 16:19:29

Conditions and loops

Conditions provide a simple but powerful way to control the flow of code execution. Loops allow you to perform repetitive operations with less code. Let's take a look at:

if conditions
switch statements
while, do-while, for, and for-in loops

Note

The examples in the following sections require you to switch to the multiline Firebug console. Or, if you use the WebKit console, use Shift + Enter instead of Enter to add a new line.

The if condition

Here's a simple example of an if condition:

var result = '', a = 3;
if (a > 2) {
  result = 'a is greater than 2';
}

The parts of the if condition are:

The if statement
A condition in parentheses—"is a greater than 2?"
A block of code wrapped in {} that executes if the condition is satisfied

The condition (the part in parentheses) always returns a Boolean value, and may also contain the following:

A logical operation: !, &&, or ||
A comparison, such as ===, !=, >, and so on
Any value or variable that can be converted to a Boolean
A combination of the above

The else clause

There can also be an optional else part of the if condition. The else statement is followed by a block of code that runs if the condition evaluates to false:

if (a > 2) {
  result = 'a is greater than 2';
} else {
  result = 'a is NOT greater than 2';
}

In between the if and the else, there can also be an unlimited number of else if conditions. Here's an example:

if (a > 2 || a < -2) {
  result = 'a is not between -2 and 2';
} else if (a === 0 && b === 0) {
  result = 'both a and b are zeros';
} else if (a === b) {
  result = 'a and b are equal';
} else {
  result = 'I give up';
}

You can also nest conditions by putting new conditions within any of the blocks:

if (a === 1) {
  if (b === 2) {
    result = 'a is 1 and b is 2';
  } else {
    result = 'a is 1 but b is definitely not 2';
  }
} else {
  result = 'a is not 1, no idea about b';
}

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Object-Oriented JavaScript（Second Edition）

Conditions and loops

Note

The if condition

The else clause