ES5 standardized String.prototype.trim. All major engines have also
implemented corresponding String.prototype.trimLeft and String.prototype.trimRight functions - without any standard specification.
Since these are defacto standard across engines, I propose that these are standardized as is.
In this discussion it was noted that the words left and right doesn't make sense for bidirectional and right-to-left languages. A more appropriately name for these functions would be trimStart and trimEnd which corresponds to the existing startsWith and endsWith.
Unfortunately, all engines already implement them and removing them would likely break the web.
See this extension to the spec.
It is my intention to propose this as a Stage 0 proposal to the next version of ECMAScript.