将 javascript 数字转换为浮点数（单精度 IEEE-754）并接收 integer 位

Question

I want to truncate a javascript number to single precision (float) and receive the integer bits. How can I do this?

For example: "5" is 0x40a00000 in IEEE-754 @ 32 bit. I want to do the 5 => 0x40a00000 mapping in Javascript.

Answer 1

This solution needs ES2015 to work and works in modern browsers and NodeJS.

Javascript numbers are doubles (IEEE-754 @ 64bit / double precision). With Float32Array we can truncate a f64 (double) value into a f32 (float) representation. With Uint32Array we can extract the raw integer bits afterwards.

Code on JSFiddle: https://jsfiddle.net/phip1611/u5b4ozks/19/

And as a Stack Snippet:

 // double is "number" (javascript data type) function jsF64ToF32IntegerBitsStr(double) { // float / f32 has 32 bit => 4 bytes const BYTES = 4; // Buffer is like a raw view into memory const buffer = new ArrayBuffer(BYTES); // Float32Array: interpret bytes in the memory as f32 (IEEE-754) bits const float32Arr = new Float32Array(buffer); // UInt32Array: interpret bytes in the memory as unsigned integer bits. // Important that we use unsigned here, otherwise the MSB would be interpreted as sign const uint32Array = new Uint32Array(buffer); // will convert double to float during assignment float32Arr[0] = double; // now read the same memory as unsigned integer value const integerValue = uint32Array[0]; // to hex string const integerBitsHex = integerValue.toString(16); // + hex prefix return '0x' + integerBitsHex; } // '0x40a00000' console.log('5 in IEEE-754 single precision is: ', jsF64ToF32IntegerBitsStr(5)); // '0xc0490625' console.log('-3.141 in IEEE-754 single precision is: ', jsF64ToF32IntegerBitsStr(-3.141)); // '0x7fffffff' console.log('NaN in IEEE-754 single precision is: ', jsF64ToF32IntegerBitsStr(NaN));

You can verify the output against this neat online tool: https://www.h-schmidt.net/FloatConverter/IEEE754.html