Equality narrowing in Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is equality narrowing in TypeScript?

Equality narrowing is a feature where TypeScript uses equality checks (like === or !==) to narrow down the possible types of a variable within a code block.

beginner

How does TypeScript narrow types after an if (x === null) check?

Inside the if block, TypeScript knows x is exactly null. Outside, it knows x is not null and narrows the type accordingly.

beginner

Given <code>let value: string | number;</code> what happens after <code>if (typeof value === 'string')</code>?

Inside the if block, TypeScript narrows value to string. Outside, it narrows value to number.

intermediate

Can equality narrowing work with custom types or only primitives?

Equality narrowing works with any types where equality checks make sense, including primitives like string, number, null, and also with literal types and enums.

intermediate

What happens if you compare two variables of union types with === in TypeScript?

TypeScript narrows the types of both variables to the overlapping types that can be equal, helping catch impossible comparisons or refine types inside the check.

What does TypeScript do after if (x === null)?

A Narrows <code>x</code> to <code>undefined</code> inside the block

B Narrows <code>x</code> to <code>null</code> inside the block

C Narrows <code>x</code> to <code>string</code> inside the block

D Does not narrow <code>x</code>

Inside the if block, TypeScript knows x must be null.

Given let val: string | number; what is val inside if (typeof val === 'string')?

A string

B boolean

C string | number

D number

The typeof check narrows val to string inside the block.

Does equality narrowing work with enum types in TypeScript?

A Only with numeric enums

B No, enums are not narrowed

C Only with string enums

D Yes, it narrows based on enum values

Equality narrowing works with enums by narrowing to specific enum values.

What happens if you compare two union type variables with ===?

A TypeScript narrows both to overlapping types

B TypeScript throws an error

C TypeScript ignores the check

D TypeScript widens the types

TypeScript narrows both variables to the types that can be equal.

Which operator triggers equality narrowing in TypeScript?

A +

B -

C ===

D *

The strict equality operator === triggers equality narrowing.

Explain how equality narrowing helps TypeScript understand variable types inside an if statement.

Think about how checking if a variable equals a value changes what TypeScript knows about it.

Describe what happens when you compare two union type variables with === in TypeScript.

Consider how TypeScript finds common types that can be equal.

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