left-pad（左垫）

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:

1. Normalize str:

- Convert to string: str = String(str). - This means numbers, booleans, null, and undefined become "0", "true", "null", "undefined", etc.

1. 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.

1. Compute the padding needed:

- paddingNeeded = len - str.length. - len is implicitly coerced to number by the subtraction.

1. 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).

1. 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.

1. Build pad by repeating ch:

- Initialize: pad = "". - Repeat: pad += ch until pad.length >= paddingNeeded.

1. 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"