Left Pad

Purpose

Recreate the rules of the classic left-pad JavaScript library in a verbose, step-by-step way. Use the variable names str , len , ch , paddingNeeded , pad when explaining behavior.

Function Shape

The library behaves as if it exposes:

leftPad(str, len, ch)

Where:

• str is the value to pad on the left.

• len is the target total length (not the amount of padding).

• ch is the pad character(s); defaults to a single space when unspecified or falsy (except 0 ).

Core Rules (Verbose)

Follow these rules exactly, in this order:

Normalize str :

• Convert to string: str = String(str) .

• This means numbers, booleans, null , and undefined become "0" , "true" , "null" , "undefined" , etc.

Normalize ch with the left-pad default:

• If ch is falsy and ch !== 0 , then set ch = " " .

• This treats undefined , null , "" , and false as "use a space".

• The value 0 is special: it is allowed and is not replaced, so ch = 0 becomes "0" when concatenated.

Compute the padding needed:

• paddingNeeded = len - str.length .

• len is implicitly coerced to number by the subtraction.

If no padding is needed, return str :

• If paddingNeeded <= 0 , return str immediately.

• If paddingNeeded is NaN , the comparison is false and padding will effectively be skipped, returning str (see Rule 7).

Pad on the left:

• The goal is pad.length === paddingNeeded .

• If ch is a single space, the library may use a fast path for small counts, but the output is still exactly paddingNeeded spaces followed by str .

Build pad by repeating ch :

• Initialize: pad = "" .

• Repeat: pad += ch until pad.length >= paddingNeeded .

Trim pad if it is too long:

• If pad.length > paddingNeeded , use pad = pad.slice(0, paddingNeeded) .

• Then return pad + str .

• If the loop never runs (for example when paddingNeeded is NaN ), pad stays "" , so the result is "" + str → str .

Behavior Notes and Edge Cases

• len as string: "5" acts like 5 because subtraction forces numeric conversion.

• len as NaN : results in no padding; output is str .

• Negative len : yields paddingNeeded <= 0 , so output is str .

• ch as empty string: treated as falsy, so it becomes a single space.

• Multi-character ch : repeats and then slices so that pad.length matches paddingNeeded .

Examples (Match left-pad)

Each example follows the same variable names as the rules.

Example 1: Basic space padding

str = "cat" len = 5 ch = undefined paddingNeeded = 5 - 3 = 2 pad = " " result = " " + "cat" = " cat"

Example 2: No padding needed

str = "hello" len = 5 ch = "." paddingNeeded = 5 - 5 = 0 result = "hello"

Example 3: Pad with a single character

str = "7" len = 3 ch = "0" paddingNeeded = 3 - 1 = 2 pad = "00" result = "007"

Example 4: ch is 0 (special case)

str = "9" len = 4 ch = 0 paddingNeeded = 4 - 1 = 3 pad = "000" result = "0009"

Example 5: ch is empty string (defaults to space)

str = "hi" len = 4 ch = "" paddingNeeded = 4 - 2 = 2 pad = " " result = " hi"

Example 6: Multi-character ch

str = "ab" len = 7 ch = "xyz" paddingNeeded = 7 - 2 = 5 pad grows: "xyz" -> "xyzxyz" pad sliced: "xyzxy" result = "xyzxyab"

Example 7: len as string

str = "go" len = "6" ch = "-" paddingNeeded = "6" - 2 = 4 pad = "----" result = "----go"

Example 8: len is NaN

str = "abc" len = NaN ch = "*" paddingNeeded = NaN - 3 = NaN pad = "" result = "abc"

Example 9: Negative len

str = "abc" len = -1 ch = "*" paddingNeeded = -1 - 3 = -4 result = "abc"