What does ? in js mean?

The question mark (?) operator in JavaScript is a conditional operator used to write conditional statements, which assigns values ​​to variables based on conditions: 1. Syntax: variable = condition ? trueValue : falseValue; 2. Usage: Simplify if-else statements, nested conditions, and implement default values. 3. Notes: right association, condition is Boolean value, object reference.

Question mark (?) operator in JavaScript

Question mark (?) operator is a conditional operator used to write conditional statements in JavaScript. It allows you to assign a value to a variable based on a certain condition.

Syntax:

<code>variable = condition ? trueValue : falseValue;</code>

Where:

• ##variable is the variable to which the value is to be assigned.
• condition is the condition to be evaluated.
• trueValue is the value to assign if the condition is true.
• falseValue is the value to assign if the condition is false.

Usage:

The question mark operator has the following usage:

• Simplified if-else statement: It can simplify if-else statements as follows:

<code>if (condition) {
  variable = trueValue;
} else {
  variable = falseValue;
}

// 等价于：
variable = condition ? trueValue : falseValue;</code>

• Nested conditions: It allows you to nest conditions as follows:

<code>variable = condition ? trueValue : (condition2 ? trueValue2 : falseValue2);</code>

• Implementing default values: It can implement default values ​​as follows:

<code>const name = user.name || "Guest"; // 如果 user.name 为 undefined 或 null，则 name 被赋予 "Guest" 值。</code>

Notes:

The question mark operator is right associative, which means it evaluates from right to left.
• Make sure the condition is a Boolean value (true or false).
• If trueValue or falseValue are objects themselves, they are referenced rather than copied.

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