Converting on purpose — Number(), String(), Boolean() and unary +

Level: 101 · for anyone turning input text into numbers

Stub — an outline, not a lesson. There is no runnable example behind this page yet, so nothing on it has been through the check that backs every other claim in this library. The questions below are what the finished page has to answer.

One line: Unary +x is Number(x) with one exception — +1n throws where Number(1n) is 1; both read "" and " " as 0 and "12px" as NaN, and String(sym) works where "" + sym throws.

Keywords: Number(), String(), Boolean(), unary +

What the finished page will answer

• What do Number(""), Number(" "), Number("12px"), Number("0x1f") and Number("1_000") return?
• Why does +1n throw a TypeError while Number(1n) returns 1?
• What do Number(null), Number(undefined), Number([]), Number([5]) and Number({}) return, and which rule produces each?
• Why does String(Symbol("id")) work while "" + Symbol("id") and a template literal throw?
• Is !!value the same as Boolean(value) for every value?

Examples it will need

• explicit_conversion_number_js.js — Number() and unary + on fifteen inputs side by side, with the one row where they differ
• explicit_conversion_string_and_boolean_js.js — String(), a template literal and Boolean() on symbols, BigInts, arrays and wrapper objects

See also

• Parsing numbers — parseInt and parseFloat, which read a prefix instead
• The + operator — the implicit conversions these replace
• Truthy and falsy — what Boolean() returns for each value
• BigInt — why a BigInt refuses unary plus
• Ruby text: to_i never fails; Integer() does ↗ — the lenient to_i and the strict Integer() in Ruby
• C: Parsing a number from text ↗ — three C parsers that fail in three different ways

Sources to start from

• MDN — Number() constructor ↗
• MDN — Unary plus (+) ↗
• ECMA-262 — StringToNumber ↗