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Avoid Using Non-existent Reversed Operators =+, =-, and =! in TypeScript

High
MaintainabilityCorrectness

What is it?​

This practice is triggered by the accidental use of operator pairs (=+, =-, or =!) that resemble their compound counterparts (+=, -=, or !=) but behave very differently. These pairs compile and run but do not perform the intended operation, which can lead to subtle bugs and confusing code.

Why apply it?​

Using these reversed operator sequences can be misleading: • They may appear to modify a variable with compounded arithmetic or comparison operations, but instead, they result in an assignment of an expression processed by a unary operator. • This misusage undermines code clarity and increases the risk of logic errors. • Correcting these issues early helps to ensure your code’s behavior matches your intent and improves maintainability.

How to Fix it?​

If your intention is to perform a compound operation, use the proper operator such as += or -=. If you intend to assign a unary negated or positive value, ensure that the operator and spacing clearly reflect that intention (for instance, having a space between the assignment operator and the unary operator).

Examples​

Example 1:​

Positive​

Correct implementation following the practice.

let temperature: number = 25;
let drop: number = 5;
temperature -= drop; // Compliant: subtracts drop from temperature correctly
console.log("New temperature:", temperature);
if (temperature < 20) {
  console.log("Temperature is getting low.");
}

Negative​

Incorrect implementation that violates the practice.

let balance: number = -100;
let deposit: number = 50;
balance =+ deposit; // Noncompliant: assigns +deposit instead of adding deposit to balance
console.log("Balance after deposit:", balance);
if (balance < 0) {
  console.log("Warning: account is overdrawn.");
}

Example 2:​

Positive​

Correct implementation following the practice.

let balance: number = -100;
let deposit: number = 50;
balance += deposit; // Compliant: correctly adds deposit to balance
console.log("Updated balance:", balance);
if (balance < 0) {
  console.log("Account is overdrawn.");
}

Example 3:​

Positive​

Correct implementation following the practice.

let userInput: string = "admin";
let expectedInput: string = "user";
if (userInput !== expectedInput) { // Compliant: uses proper inequality operator for comparison
  console.log("User input does not match the expected input.");
} else {
  console.log("User input matches.");
}
console.log("Comparison complete.");

Negative​

Incorrect implementation that violates the practice.

let temperature: number = 25;
let drop: number = 5;
temperature =- drop; // Noncompliant: assigns -drop instead of subtracting drop from temperature
console.log("Temperature after drop:", temperature);
if (temperature < 20) {
  console.log("Alert: temperature is critically low.");
}

Example 4:​

Negative​

Incorrect implementation that violates the practice.

let userInput: string = "admin";
let expectedInput: string = "user";
let isDifferent: boolean;
isDifferent =! (userInput === expectedInput); // Noncompliant: confusing usage, assigns the negation of the comparison result
if (isDifferent) {
  console.log("User input is different from expected.");
} else {
  console.log("User input matches.");
}
console.log("Check completed.");