String coercion

Certain methods like concat will coerce their argument values to a string type before operating on them.

However, TypeScript will often enforce a more restrictive type on the arguments to these functions.

Rules of string coercion

• undefined becomes "undefined" and null becomes "null"
• true becomes "true", and false becomes "false"
• Strings are returned as-is
• Numbers and BigInt are converted with .toString(10) (i.e. decimal representation)
• Symbols will throw a TypeError
• Objects are first converted to a primitive value by calling [Symbol.toPrimitive](), in most cases with "string" as a hint.

If a conversion function is not defined, the primitive value is retrieve from toString() or valueOf(), in that order. The resulting primitive value is then converted to a string.

Adding an empty string

It is common to see JavaScript programs that coerce a number to a string by adding the empty string "".

For readability reasons alone, it is better to use the String function or toString to convert a number to a string.