if...else

當條件成立的時候會執行 if 陳述式裡的程式，而不成立時則執行另外一個陳述式。

這個互動式的源碼被放在 GitHub 的 Repository 裡，如果您想對此互動式範例專案提出貢獻的話，請 clone https://github.com/mdn/interactive-examples 並送出 pull request。

語法

if (條件式)
   statement1
[else
   statement2]

條件式

一個成立或不成立的運算式。

statement1

Statement that is executed if condition is truthy. Can be any statement, including further nested if statements. To execute multiple statements, use a block statement ({ ... }) to group those statements. To execute no statements, use an empty statement.

statement2

Statement that is executed if condition is falsy and the else clause exists. Can be any statement, including block statements and further nested if statements.

Description

Multiple if...else statements can be nested to create an else if clause. Note that there is no elseif (in one word) keyword in JavaScript.

if (condition1)
   statement1
else if (condition2)
   statement2
else if (condition3)
   statement3
...
else
   statementN

To see how this works, this is how it would look if the nesting were properly indented:

if (condition1)
   statement1
else
   if (condition2)
      statement2
   else
      if (condition3)
...

To execute multiple statements within a clause, use a block statement ({ ... }) to group those statements. In general, it is a good practice to always use block statements, especially in code involving nested if statements:

if (condition) {
   statements1
} else {
   statements2
}

Do not confuse the primitive boolean values true and false with truthiness or falsiness of the Boolean object. Any value that is not false, undefined, null, 0, NaN, or the empty string (""), and any object, including a Boolean object whose value is false, is considered truthy when used as the condition. For example:

var b = new Boolean(false);
if (b) // this condition is truthy

Examples

Using if...else

if (cipher_char === from_char) {
   result = result + to_char;
   x++;
} else {
   result = result + clear_char;
}

Using else if

Note that there is no elseif syntax in JavaScript. However, you can write it with a space between else and if:

if (x > 5) {
 /* do the right thing */
} else if (x > 50) {
 /* do the right thing */
} else {
 /* do the right thing */
}

Assignment within the conditional expression

It is advisable to not use simple assignments in a conditional expression, because the assignment can be confused with equality when glancing over the code. For example, do not use the following code:

if (x = y) {
   /* do the right thing */
}

If you need to use an assignment in a conditional expression, a common practice is to put additional parentheses around the assignment. For example:

if ((x = y)) {
   /* do the right thing */
}

Specifications

Browser compatibility

See also

• block
• switch
• conditional operator