Example

var sentence = document.createTextNode
    ("When you're moaning, the whole world smiles with you.");

sentence.replaceData(12, 7, 'smiling');

In the example above we create a Text node with the value When you're moaning, the whole world smiles with you., then we use the replaceData method to replace seven characters starting from character offset 12 (moaning) with smiling.

That operation changes the Text node so it now has the data (and nodeValue) When you're smiling, the whole world smiles with you., and a length of 53.

Then if we perform an additional replacement, starting from character offset 21 of the modified node, until the end of the string (the second argument is longer than the total length of the string, which will replace everything from the initial offset to the end), and replace that with the string the sun comes shining through.:

sentence.replaceData(21, 999, 'the sun comes shining through.');

We'll finally end up with a Text node that has the data (and nodeValue) When you're smiling, the sun comes shining through., and a length of 51.