#字串運算

> x = "hello"
'hello'
> x.length
5
> x[3]
'l'
> x[2]
'l'
> x[1]
'e'
> x[0]
'h'
> x[5]
undefined
> x[4]
'o'
> x=x+" world"
'hello world'
> x
'hello world'
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