JavaScript SyntaxError – Test for equality (==) mistyped as assignment (=)?

This JavaScript warning test for equality (==) is mistyped as an assignment (=)? occurs if by assignment (=) is used in place of equality (==).

Message:

Warning: SyntaxError: test for equality (==) mistyped as assignment (=)?

Error Type:

SyntaxError: Warning which is reported only if 
javascript.options.strict preference is set to true.

Cause of error: There is an assignment (=) used in place of equality (==) in the code. 

Example 1: In this example, the ‘=’ is used in place of ‘==’. So the error has occurred.

Javascript

if (a = b) { // Error here
    // do something
}

Output:

Warning: SyntaxError: test for equality (==) 
mistyped as assignment (=)?

Example 2:  In this example, the ‘=’ is used in place of ‘==’. So the error has occurred.

Javascript

let a = 5;
let b = 4;
let c = 5;
if (b = c) {
    // do something
} else if (a = c) {
    // do something
}

Output:

Warning: SyntaxError: test for equality (==) 
mistyped as assignment (=)?