Let `x` be a number. How does `x.toString()` work?

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This is defined in the ECMAScript spec. However, the language of the spec is rigorous and not straightforward to read. Here’s a plain English explanation for the case in which radix = 10 (i.e., when you call x.toString() with no additional parameter):

  1. If x is NaN, x.toString() returns "NaN".
  2. Else if x is zero (+0 or -0), x.toString() returns "0".
  3. Else if x is negative, x.toString() returns the string concatenation of "-" and (-x).toString().
  4. Else if x is Infinity, x.toString() returns "Infinity".
  5. Else if x>=1e-6 and x<1e21, x.toString() returns the fixed-point notation of x. For example:
    • (100).toString() returns "100".
    • 100.11.toString() returns "100.11".
    • 1e-6.toString() returns "0.000001".
  6. Else (here, x must be positive and not in the range specified in 5), x.toString() returns the scientific notation of x. For example:
    • 1e21.toString() returns "1e+21".
    • 9.9999999999999e-7.toString() returns "9.9999999999999e-7".