String replace function behavior - error "nothing to repeat"

Because {n} (with n being a positive integer) is quantifier syntax, see https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Regular_Expressions/Quantifiers

You don’t want the curly braces to have their “special meaning” here, but want them treated as plain characters - so you should escape them.

a.replace(/\{0\}/g,"little")

Your argument to replace() is a regular expression.

In particular, braces { and } are regular expression metacharacters (they are a range quantifier).

You must escape (with \) the braces in your regular expression for this to work.

For example, this:

a.replace(/\{0\}/g, "little");

would work. This would treat the braces literally.

The specific error message you get:

SyntaxError: Invalid regular expression: /{0}/: Nothing to repeat

is because the engine expects some text to precede the quantifier, so for example, x{0} would be a valid argument as a regular expression.

In the other case, using

a.replace(/{str}/g, "little");

does work, since the braces lose their special meaning as metacharacters, and get treated literally. Range quantifiers must take the form {n}, {n,} or {n,m} where n and m are integers.

Problem

The number inside a curly brace has a special meaning, it is quanitifier syntax.

Solution

You have to escape the curly braces with \{ \}

Here is an example

var b = "hello {0} world"; 
console.log(b.replace(/\{0\}/g,"little"));