gistfile1.js

String.prototype.c = function (b) { console.log(this); return this.concat(b) }
String.c(b)

"a".c("b")
VM2531:2 String {0: "a", length: 1, [[PrimitiveValue]]: "a"}
"ab"

("a".c)("b")
VM2531:2 String {0: "a", length: 1, [[PrimitiveValue]]: "a"}
"ab"

d = "a".c
String.c(b)

d("b")
VM2531:2 Window {top: Window, location: Location, document: document, window: Window, external: Object…}
VM2531:2 Uncaught TypeError: this.concat is not a function
    at String.c (<anonymous>:2:68)
    at <anonymous>:2:1
    at Object.InjectedScript._evaluateOn (<anonymous>:895:140)
    at Object.InjectedScript._evaluateAndWrap (<anonymous>:828:34)
    at Object.InjectedScript.evaluate (<anonymous>:694:21)
    
function i(x) { return x }
undefined

i("a".c)("b")
VM2531:2 Window {top: Window, location: Location, document: document, window: Window, external: Object…}
VM2531:2 Uncaught TypeError: this.concat is not a function
    at String.c (<anonymous>:2:68)
    at <anonymous>:2:9
    at Object.InjectedScript._evaluateOn (<anonymous>:895:140)
    at Object.InjectedScript._evaluateAndWrap (<anonymous>:828:34)
    at Object.InjectedScript.evaluate (<anonymous>:694:21)

("a".concat.valueOf())("b")
VM507:2 Uncaught TypeError: String.prototype.concat called on null or undefined
    at concat (native)
    at <anonymous>:2:23
    at Object.InjectedScript._evaluateOn (<anonymous>:895:140)
    at Object.InjectedScript._evaluateAndWrap (<anonymous>:828:34)
    at Object.InjectedScript.evaluate (<anonymous>:694:21)

x = "a".concat.valueOf()
concat()

x("b")
VM211:2 Uncaught TypeError: String.prototype.concat called on null or undefined
    at concat (native)
    at <anonymous>:2:1
    at Object.InjectedScript._evaluateOn (<anonymous>:895:140)
    at Object.InjectedScript._evaluateAndWrap (<anonymous>:828:34)
    at Object.InjectedScript.evaluate (<anonymous>:694:21)(anonymous function) @ VM211:2InjectedScript._evaluateOn @ VM76:895InjectedScript._evaluateAndWrap @ VM76:828InjectedScript.evaluate @ VM76:694

x.call("a","b")
"ab"