Explicit function length definition is not necessary for the following methods:
19.2.3.1 Function.prototype.apply ( thisArg, argArray )
20.1.3.2 Number.prototype.toExponential ( fractionDigits )
20.1.3.3 Number.prototype.toFixed ( fractionDigits )
20.1.3.5 Number.prototype.toPrecision ( precision )
20.3.3.4 Date.UTC ( year, month [ , date [ , hours [ , minutes [ , seconds [ , ms ] ] ] ] ] )
20.3.4.21 Date.prototype.setFullYear ( year [ , month [ , date ] ] )
20.3.4.22 Date.prototype.setHours ( hour [ , min [ , sec [ , ms ] ] ] )
20.3.4.24 Date.prototype.setMinutes ( min [ , sec [ , ms ] ] )
20.3.4.25 Date.prototype.setMonth ( month [ , date ] )
20.3.4.26 Date.prototype.setSeconds ( sec [ , ms ] )
20.3.4.29 Date.prototype.setUTCFullYear ( year [ , month [ , date ] ] )
20.3.4.30 Date.prototype.setUTCHours ( hour [ , min [ , sec [ , ms ] ] ] )
20.3.4.32 Date.prototype.setUTCMinutes ( min [ , sec [, ms ] ] )
20.3.4.33 Date.prototype.setUTCMonth ( month [ , date ] )
20.3.4.34 Date.prototype.setUTCSeconds ( sec [ , ms ] )
21.1.1.1 String ( value )
21.1.3.16 String.prototype.slice ( start, end )
21.1.3.17 String.prototype.split ( separator, limit )
21.1.3.19 String.prototype.substring ( start, end )
21.2.5.11 RegExp.prototype [ @@split ] ( string, limit )
22.1.3.12 Array.prototype.join (separator)
22.1.3.22 Array.prototype.slice (start, end)
22.2.3.23 %TypedArray%.prototype.slice ( start, end )
22.2.3.26 %TypedArray%.prototype.subarray( [ begin [ , end ] ] )
B.2.3.1 String.prototype.substr (start, length)
Fixed in ES2016 Draft.