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padStart and trim

Learn string formatting with .padStart and .trim.

Padding with padStart

To pad a string to a specific length you use .padStart(targetLength, padString). The string is prefixed with padString until the target length is reached. "5".padStart(3, "0") returns "005".

padStart is useful for showing codes, phone or sequence numbers at a fixed width. For example "42".padStart(6, "0") returns "000042"; this way every code in a list lines up at the same width. Its counterpart .padEnd() pads at the end.

Trimming whitespace with trim

To remove leading and trailing whitespace (space, tab, newline) from a string you use .trim(). " alex ".trim() returns "alex". Whitespace often appears around user input; trim removes it in one call.

To remove only leading whitespace use .trimStart(), only trailing use .trimEnd(). trim does not modify the original string; it returns a new one. Trimming fields like email or username makes comparisons reliable.

Practice Pad the code to 6 characters

The code variable holds "42". Declare a variable padded and assign the result of code.padStart(6, "0") to it. Then print padded with console.log. The output on a single line must be 000042.

Starter code

index.js
const code = "42";
// Declare padded with padStart and print it

Sample solution

index.js
const code = "42";
const padded = code.padStart(6, "0");
console.log(padded);

Expected output

000042

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 Write the padStart call

Question 
const code = "5";
const padded = code____(3, "0");
Answer 
const code = "5";
const padded = code.padStart(3, "0");

Explanation: .padStart pads at the start of the string.

2 What is the result of "7".padStart(4, "0")?

index.js
"7".padStart(4, "0")
  • "7"
  • "7000"
  • "0007" Correct answer
  • "0"

Explanation: padStart(4, "0") prepends three zeros; the result is "0007".

3 Match each method to its job

Term Meaning
.padStart(n, x) Pads at the start
.padEnd(n, x) Pads at the end
.trim() Removes side whitespace
.trimStart() Removes only the start

All lessons in Beginner

1 Node.js basics

  1. What is Node.js?
  2. Your first program
  3. ES modules
  4. package.json
  5. CommonJS vs ESM
  6. Summary - Node.js basics

2 Variables

  1. let and const
  2. var and scope
  3. Type inference
  4. Destructuring
  5. Spread and rest
  6. Summary - Variables

3 Data types

  1. Primitive types
  2. Template literals
  3. Number and Math
  4. Symbol
  5. typeof
  6. Summary - Data types

4 Project: Calculator

  1. Project design
  2. Add and subtract
  3. Multiply and divide
  4. process.argv
  5. Putting it together

5 Operators

  1. Arithmetic operators
  2. Assignment operators
  3. Comparison operators
  4. Logical operators
  5. Nullish coalescing
  6. Summary - Operators

6 Strings

  1. String methods
  2. String search
  3. substring and slice
  4. replace and split
  5. padStart and trim
  6. Summary - Strings

7 Conditionals

  1. The if statement
  2. else and else if
  3. switch
  4. Ternary
  5. Optional chaining
  6. Summary - Conditionals

8 Loops

  1. The for loop
  2. The while loop
  3. for...of
  4. for...in
  5. break and continue
  6. Summary - Loops

9 Project: Shopping cart

  1. Cart design
  2. Adding items
  3. Calculating totals
  4. Listing the cart
  5. Putting it together

10 Arrays

  1. Declaring arrays
  2. Array access
  3. push and pop
  4. shift and unshift
  5. splice and slice
  6. Summary - Arrays

11 Array methods

  1. map
  2. filter
  3. reduce
  4. find and some
  5. forEach
  6. Summary - Array methods

12 Objects

  1. Object literal
  2. Property access
  3. Computed properties
  4. Object methods
  5. Object spread
  6. Summary - Objects

13 Functions

  1. Defining functions
  2. Return values
  3. Arrow functions
  4. Default parameters
  5. Rest parameters
  6. Summary - Functions

14 Callbacks

  1. What are callbacks?
  2. Higher-order functions
  3. forEach with callbacks
  4. Error-first callbacks
  5. Callback hell
  6. Summary - Callbacks

15 Project: To-do CLI

  1. Task model
  2. Adding tasks
  3. Listing tasks
  4. Completing tasks
  5. Putting it together

16 Promises

  1. What are promises?
  2. then and catch
  3. Promise.all
  4. Promise.race
  5. finally
  6. Summary - Promises

17 async/await

  1. async function
  2. await
  3. try/catch
  4. await with Promise.all
  5. Sequential await
  6. Summary - async/await

18 fs module

  1. fs.promises
  2. readFile
  3. writeFile
  4. appendFile
  5. existsSync
  6. Summary - fs module

19 path module

  1. path.join
  2. path.resolve
  3. path.parse
  4. path.dirname and basename
  5. path.extname
  6. Summary - path module

20 Beginner final

  1. Project design
  2. Data model
  3. Business logic
  4. fs persistence
  5. Putting it together