Are trailing commas in arrays and objects part of the spec?

Question

Are trailing commas standard in JavaScript, or do most browsers like Chrome and Firefox just tolerate them?

I thought they were standard, but IE8 puked after encount

Answer 1

On Chrome 52 :

[1,].length --> 1
[1,2,].length --> 2
[].length --> 0 
[,].length --> 1    <<<<=== OUHHHHH !!!!

I just don't like trailing commas. People usually use them in Open Source projects for avoiding to erase the line written by another commiter. See the same question in Python : https://stackoverflow.com/a/11597911/968988