What are logical assignment operators (&&=, ||=, ??=)?

Logical assignment operators in JavaScript include &&=, ||= and ??=, which are assigned when the variable is true, false, null, or undefined, respectively. 1. &&=: Assign value only when the variable is true, and it is suitable for updating its content when there is already a valid value; 2. ||=: Assign value only when the variable is false, it is often used to set default values ​​or supplement missing data; 3. ??=: Assign value only when the variable is null or undefined, it is suitable to safely initialize variables without overwriting legal values. These operators are suitable for modern environments and are recommended for concise assignment scenarios to improve code clarity, but pay attention to readability and adaptability of teamwork.

Logical assignment operators ( &&= , ||= , ??= ) are newer features in JavaScript. They combine logical operations and assignment operations to make the code simpler and the intent clearer. These operators are particularly convenient when handling conditional assignments, and are especially suitable for further processing when the variable already has a value.

1. &&= : Assign value only when the variable is true

The function of this operator can be understood as:

If the variable on the left is a "truthy", then assign the value on the right to it.

For example:

 let x = 5;
x &&= 10;
console.log(x); // Output 10

Here, because x is 5 (a true value), x is assigned to 10 .

But if the initial value is a false value (such as 0 , null , undefined , false , '' etc.), the assignment will not be performed:

 let y = 0;
y &&= 7;
console.log(y); // Output 0

Common uses:

• Modify a value if it exists.
• Avoid redundant operations on invalid values.

2. ||= : Assign value only if the variable is false

This operator means:

If the variable on the left is a "false value", then assign the value on the right to it.

Example:

 let a = null;
a ||= 'default';
console.log(a); // Output 'default'

And if the variable already has a valid value, it will not be changed:

 let b = 'already value';
b ||= 'alternative value';
console.log(b); // Output 'already value'

Common uses:

• Set default values, especially function parameters or configuration items.
• Supplement missing data fields.

3. ??= : Assign value only if the variable is null or undefined

This is the most "strict" one, and its rules are:

The right value will only be assigned to it if the value on the left is null or undefined .

Unlike the other two, it does not care about other false values ​​(such as empty strings, 0, false), but only focuses on whether it is really "undefined".

Example:

 let c = undefined;
c ??= 'supplementary value';
console.log(c); // Output 'supplementary value'

let d = '';
d ??= 'supplementary value';
console.log(d); // Output ''

Common uses:

• Handles possible undefined object properties.
• Safely initialize variables without overwriting legal values ​​(including 0 and empty strings).

Recommendations and precautions for use

• These operators are suitable for modern browsers and Node.js environments (ES2020).
• Don't overuse, especially in teamwork, make sure everyone is familiar with these syntaxes.
• Try to use it in concise assignment scenarios rather than complex logical judgments.

For example, the following situations are very suitable:

• Set default values ​​for variables (especially object properties)
• Decide whether to update based on whether the current value exists
• Clearly express the intention of "assigning values ​​only under certain conditions"

Basically that's it. The rational use of logical assignment operators can make the code cleaner, but you should also pay attention to readability and avoid writing confusing "one-line code".

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