The languages have enough in common to make learning one easy if you know the other. By the same token, the differences are subtle enough to trip up those proficient in both. Here's a short list comparing C and JavaScript:
Terminating JavaScript command lines in semicolons is optional; in C it's mandatory. Recommended practice is to use them religiously in both languages (and Java as well).
Both JavaScript and C are case-sensitive; 'doThis' is different from 'DOTHIS'. Experienced programmers learn to love this feature, which drives beginners nuts.
Both JavaScript and C are block-structured computer languages and employ curly brackets -- '{' and '}' -- to delimit blocks.
Both JavaScript and C employ quotation -- enclosure in single or double quote marks -- to designate text strings.
Arrays in both JavaScript and C are zero-based; the first element is myArray[0], not myArray[1].
Both JavaScript and C employ '==' for comparison, '=' for equality, and '!' for negation. In fact the set of JavaScript operators is essentially borrowed from C (right down to the deprecated ternary construct a ? b : c).
Both JavaScript and C employ the symbols /* to designate a comment */. JavaScript also permits the use of '//' for short comments, as in C++.
Finally, JavaScript's statements are a strict subset of C++'s, offering a smaller selection of identical looping and conditional constructs.