Using this calculator, I can see how the decimal \".8\" and the expression \".7

Question

Using this calculator, I can see how the decimal ".8" and the expression ".7 + .1" have different representations:

.8      = 1.1001100110011001100110011001100110011001100110011010 *2-1
.7 + .1 = 1.1001100110011001100110011001100110011001100110011001 *2-1
                                                              ^^
But what mechanism causes "0.8" to be printed for the top value? E.g. alert(.8) in JavaScript. Why does it not print something like "0.800000011920929"?

Is this a feature of IEEE 754 or the programming language implementing it?

Explanation / Answer

The paper How to Print Floating-Point Numbers Accurately by Guy L. Steele Jr. and Jon L White describes one approach to the problem of printing numbers.

Quoting from that paper:

What is the correct number of digits to produce if the user doesn

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