E139: That isn't a valid number

A decimal number has exactly one ., with at least one digit after it:

mut pi: double = 3.14;mut half: double = .5;           // the digit before the point is optionalmut big: double = 6.022e23;      // the point comes before any exponent

Turf found a . in a place a number can’t have one. The message says which of three it is.

A second decimal point. 3.14.15 looks like two numbers run together – 3.14 and .15 – and Turf won’t guess which one you meant:

mut x: double = 3.14.15;         // ERROR: a number can have only one '.'mut y: double = 1e5.5;           // ERROR: the point goes BEFORE the exponent

If you meant one decimal, remove a point. If you meant two values, put a comma or an operator between them: 3.14 + 0.15.

A point with nothing after it. 5. is not a number, because a decimal needs a digit after the point:

mut x: double = 5.;              // ERROR: '5.' ends in a '.'

Write 5.0 for the decimal or 5 for the whole number.

A point on a hexadecimal, binary or octal number. Those are whole numbers, so they have no fractional part:

mut x: double = 0xFF.5;          // ERROR: hexadecimal numbers are whole numbers

Write the value as an ordinary decimal instead (255.5).

What is not an error: 1..5 is a range, not 1. and .5, and a letter right after the point is a method call on the value (5.abs() is abs called on 5), so neither is touched by this check.

Run turf --explain E139 in a terminal to read this again.