Equality narrowing in Typescript

Introduction

Equality narrowing helps TypeScript understand what type a variable really is by checking if it equals something else. This makes your code safer and easier to read.

When you have a variable that can be more than one type and you want to do something only if it is a specific type.

When you want to avoid errors by making sure a value is what you expect before using it.

When you want TypeScript to help you by narrowing down types after checking equality.

When you write functions that accept different types and behave differently based on the input.

When you want clearer code that shows exactly what types are handled in each part.

Syntax

Typescript

if (variable === someValue) {
  // TypeScript knows variable is the type of someValue here
}

Use === or !== to compare values for equality or inequality.

TypeScript narrows the type inside the if block based on the comparison.

Examples

Checks if id is a string or number and narrows the type accordingly.

Typescript

function printId(id: number | string) {
  if (typeof id === "string") {
    console.log(id.toUpperCase());
  } else {
    console.log(id.toFixed(2));
  }
}

Narrowing by checking if x equals a specific string.

Typescript

function checkValue(x: string | number) {
  if (x === "hello") {
    console.log("x is the string 'hello'");
  } else {
    console.log("x is not 'hello'");
  }
}

Using !== to narrow down the possible values.

Typescript

function example(value: "yes" | "no" | "maybe") {
  if (value !== "maybe") {
    console.log("Value is yes or no");
  } else {
    console.log("Value is maybe");
  }
}

Sample Program

This program uses equality narrowing to check the input and print different messages based on its value and type.

Typescript

function describe(input: number | string) {
  if (input === 42) {
    console.log("The answer to life!");
  } else if (typeof input === "string" && input === "hello") {
    console.log("A friendly greeting.");
  } else {
    console.log("Just something else.");
  }
}

describe(42);
describe("hello");
describe(10);

Important Notes

Equality narrowing works best with literal values or types that can be compared directly.

TypeScript can narrow types inside if, else if, and else blocks based on your checks.

Use strict equality (===) to help TypeScript understand your intent clearly.

Summary

Equality narrowing lets TypeScript know the exact type after checking equality.

Use === or !== to compare values and narrow types safely.

It helps write safer and clearer code by handling different types properly.

Practice

1. What does equality narrowing in TypeScript help you do?

easy

A. It lets TypeScript know the exact type after checking equality.

B. It automatically converts types without checks.

C. It disables type checking for variables.

D. It only works with numbers, not strings.

Solution

1. Step 1: Understand equality narrowing concept

   Equality narrowing means TypeScript uses equality checks (like ===) to know the exact type of a variable.

2. Step 2: Compare options with definition

   Only It lets TypeScript know the exact type after checking equality. correctly describes this behavior. Others are incorrect or unrelated.

3. Final Answer:

   It lets TypeScript know the exact type after checking equality. -> Option A

4. Quick Check:

   Equality narrowing = exact type after equality check [OK]

Hint: Equality narrowing means type known after === check [OK]

Common Mistakes:

• Thinking it converts types automatically
• Believing it disables type checking
• Assuming it only works for numbers

2. Which of the following is the correct syntax to check equality for narrowing in TypeScript?

easy

A. if (value === 10) { }

B. if (value = 10) { }

C. if (value == 10) { }

D. if (value != 10) { }

Solution

1. Step 1: Identify correct equality operator for narrowing

   TypeScript uses strict equality === for safe type narrowing.

2. Step 2: Check each option

   if (value === 10) { } uses ===, which is correct. if (value == 10) { } uses loose equality, which is less safe. if (value = 10) { } is assignment, not comparison. if (value != 10) { } is inequality, not equality.

3. Final Answer:

   if (value === 10) { } -> Option A

4. Quick Check:

   Use === for equality narrowing [OK]

Hint: Use === for equality narrowing, not == or = [OK]

Common Mistakes:

• Using = instead of ===
• Using == which is not strict
• Confusing != with ===

3. What will be the output of this TypeScript code?

function check(x: string | number) {
  if (typeof x === 'string') {
    return x.toUpperCase();
  } else {
    return x.toFixed(2);
  }
}
console.log(check(3.14159));

medium

A. "3.14" (error)

B. "3.14" (string)

C. "3.14"

D. "3.14" (number)

Solution

1. Step 1: Understand type narrowing in function

   The function checks if x is a string or number using typeof. For number, it calls toFixed(2).

2. Step 2: Evaluate output for input 3.14159

   Since input is number, it returns 3.14159.toFixed(2) which is string "3.14".

3. Final Answer:

   "3.14" -> Option C

4. Quick Check:

   Number input returns toFixed string "3.14" [OK]

Hint: toFixed returns string, so output is "3.14" [OK]

Common Mistakes:

• Thinking toFixed returns number
• Confusing string and number output
• Ignoring type narrowing with typeof

4. Identify the error in this TypeScript code using equality narrowing:

function process(value: string | number) {
  if (value === 'hello') {
    return value.toUpperCase();
  } else {
    return value.toUpperCase();
  }
}

medium

A. Error: should use == instead of ===.

B. No error, code is correct.

C. Error: missing else if for number type.

D. Error: calling toUpperCase on number in else block.

Solution

1. Step 1: Analyze type narrowing with equality check

   The check value === 'hello' narrows value to string only if it equals 'hello'. Else block still has value as string or number.

2. Step 2: Identify error in else block

   Calling toUpperCase() on value in else block is unsafe because value could be number, causing runtime error.

3. Final Answer:

   Error: calling toUpperCase on number in else block. -> Option D

4. Quick Check:

   toUpperCase on number causes error [OK]

Hint: Check types before calling string methods [OK]

Common Mistakes:

• Assuming else block is string only
• Using == instead of === for narrowing
• Ignoring runtime errors from wrong method calls

5. Given this TypeScript code, what is the correct way to safely handle both types using equality narrowing?

function formatValue(val: string | number | null) {
  if (val === null) {
    return "No value";
  } else if (typeof val === 'string') {
    return val.trim();
  } else {
    return val.toFixed(1);
  }
}

hard

A. Missing check for undefined, so unsafe.

B. Code is correct and safely narrows all types.

C. Should use == null instead of === null for null check.

D. toFixed should be called only after checking val is number.

Solution

1. Step 1: Check null handling

   The first if checks val === null, safely handling null case.

2. Step 2: Check type narrowing for string and number

   The else if uses typeof val === 'string' to narrow string, else block handles number with toFixed(1).

3. Step 3: Confirm all cases covered

   All possible types (null, string, number) are handled safely with proper narrowing.

4. Final Answer:

   Code is correct and safely narrows all types. -> Option B

5. Quick Check:

   All types handled safely with === and typeof [OK]

Hint: Check null first, then typeof for string, else number [OK]

Common Mistakes:

• Forgetting null check
• Using == null which includes undefined
• Calling toFixed without type check