获得绝对价值: Math.abs()
document.write(Math.abs(7.25)); // 7.25
document.write(Math.abs(-7.25)); // 7.25
document.write(Math.abs(null)); // 0
document.write(Math.abs("Hello")); // NaN
document.write(Math.abs(7.25-10)); // 2.75
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