JavaScript SyntaxError – Invalid left-hand side in postfix operation

A "SyntaxError: Invalid left-hand side in postfix operation" error occurs when JavaScript’s interpreter comes across an invalid expression that appears on the left hand side of a postfix increment (++) or decrement (--) operator, it is only variables or expressions which can take new values after this operation that are supposed to be used with postfix operators, let us examine this error, its causes and how it can be resolved using examples.

Understanding an error

An “Invalid left-hand side in postfix operation” error means that the expression on the left-hand side of a postfix increment or decrement operator (++ or --) is not a legal target for the modification, that variable, object property, or array element must be directly incremented or decremented by JavaScript, let's some cases where this error can occur and how to resolve this error.

Case 1: Error Cause: Using a Literal or Constant as Left-Hand Side

If we try to do postfix increment or decrement operation for literal or constant it will give an "Invalid left-hand side in postfix operation" error.

Example:

const num = 5;
num++;
console.log(num);

Output:

SyntaxError: Invalid left-hand side in postfix operation

Resolution of error

The use of variable names that can be re-assigned is what makes the postfix increment or decrement operator (++ or --) helpful.

let num = 5;
num++;
console.log(num); 

6

Case 2: Error Cause: Using an Expression as Left-Hand Side

If you use a complex expression or function call on the left side of a postfix increment or decrement operation then you may run into this error.

Example:

let index = 0;
(index + 1)++;
console.log(index);

Output:

SyntaxError: Invalid left-hand side in postfix operation

Resolution of error

Postfix operations (++) should only involve simple variables not complex expressions like function calls.

let index = 0;
index++;
console.log(index); 

1

Conclusion

In conclusion if you want to prevent “Invalid left-hand side in postfix operation” errors in JavaScript, make sure to use valid targets that can be directly modified when you apply the postfix increment (++) and decrement (--) operators, these might include but are not limited to variables, object properties, and array elements, this approach promotes code coherence while at the same time adhering to JavaScript syntax rules governing use of post-fix operations.

pankajbind

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Article Tags :

• JavaScript
• Web Technologies
• JavaScript-Errors