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This method returns a string representing the number to a specified precision:

new Number(21.2).toPrecision(0) //error! argument must be > 0
new Number(21.2).toPrecision(1) //2e+1 (= 2 * 10^1 = 2)
new Number(21.2).toPrecision(2) //21
new Number(21.2).toPrecision(3) //21.2
new Number(21.2).toPrecision(4) //21.20
new Number(21.2).toPrecision(5) //21.200