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The difference between substr() and substring() in JavaScript is that substr() is used to extract a specified number of characters from a given string, starting from a specified position, while substring() is used to extract the characters from a string between two specified indices. substr() can accept negative index values, while substring() cannot.

See below example

var str = "Hello world!";
var res = str.substr(1, 4);

console.log(res); // ello

var str = "Hello world!";
var res = str.substring(1, 4);

console.log(res); // ell