TypeScript Error Fix: The Types Have No Overlap

Updated: January 8, 2024 By: Guest Contributor Post a comment


TypeScript is a powerful superset of JavaScript that introduces types into the JavaScript ecosystem. However, as developers work with types, they may encounter an error message: “Condition will always return ‘true’ since the types have no overlap.” This error typically occurs when trying to compare two variables whose types have no possible overlap, meaning there’s no set of values for which the condition could be true.

Solution 1: Type Assertion

Type assertion is a way in TypeScript to tell the compiler, ‘Trust me, I know what I’m doing.’ It’s like a type cast in other languages but doesn’t perform a special check or restructuring of data. It has no runtime impact and is only used by the compiler.

  • Step 1: Identify the variable and the condition causing the error.
  • Step 2: Perform a type assertion using angle-bracket syntax or the ‘as’ keyword.
  • Step 3: Rewrite the condition to use the asserted type.


const a: unknown = 'Test';
if ((a as string) === 'Test') {
    console.log('a is a string');

Pros: Quick and straightforward method to resolve the error when you are confident about the type.

Cons: It could lead to runtime errors if the type assertion is incorrect.

Solution 2: Type Guards

Type guards are a TypeScript feature that allow you to narrow down the type of a variable within a conditional block.

  • Step 1: Understand the kind of types you’re dealing with and determine if a common property exists.
  • Step 2: Implement a type guard function to check for the specific property.
  • Step 3: Use the type guard function in the condition.


function isString(value: unknown): value is string {
    return typeof value === 'string';
const a: unknown = 'Test';
if (isString(a)) {
    console.log('a is a string');

Benefits: Provides runtime type checking which reduces the chance of errors.

Limitations: Needs extra effort and code to define type guard functions.

Solution 3: Type Narrowing with Control Flow Analysis

TypeScript’s control flow analysis can automatically narrow types in certain situations based on control flow constructs like if blocks, switch cases, and loops.

  • Step 1: Get familiar with the control flow situations where TypeScript can automatically narrow down types.
  • Step 2: Refactor the code to take advantage of control flow-based type narrowing.


const a: unknown = 'Test';
if (typeof a === 'string') {
    console.log('a is a string');

Advantages: Utilizes TypeScript’s built-in type narrowing without extra code.

Limitations: It may not always work for more complex conditions or custom types without the user-written type guards.


In summary, the TypeScript error message indicating that the types have no overlap challenges the developer to correctly align the types within a conditional comparison. The suggested solutions, including type assertion, type guards, and control flow-based type narrowing, each have their nuances and are situationally beneficial. The choice of solution can depend on the specific circumstances, desired level of type safety, and the complexity of the codebase.

Understanding and correctly applying these techniques will lead to more robust and maintainable TypeScript codes. Therefore, having multiple tools to handle type-related issues allows developers to navigate through any type challenge, combining both flexibility and type safety provided by TypeScript.