How to get frequency spectrum from an audio wav file using fft.js with node.js

Question

I'm trying to extract frequency information of simple 16 bit pcm wav file containing only one pure tone (750hz created with audacity) in JavaScript with nodejs.

I'm using:

• node-wav for reading the wav file
• fft.js to perfom fft operation
• nodeplotlib to finaly plot the results

Here is the code i came up with:

const wav = require('node-wav');
const FFT = require('fft.js');
const fs = require('fs');
const plotlib = require('nodeplotlib');


let size = 4096; //fft size
let fft = new FFT(size); //create fft object
let realOutput = new Array(size); // to store final result
let complexOutput = fft.createComplexArray(); // to store fft output


let buffer = fs.readFileSync('750hz.wav'); // open a 1s wav file(mono 16bit pcm file at   32000hz) containing only a 750hz sinusoidal tone
let result = wav.decode(buffer); // read wav file data
let audioData = Array.prototype.slice.call( result.channelData[0]); // convert Float32Array to normal array
realInput = audioData.slice(0,size); // use only 4096 sample from the buffer.

fft.realTransform(complexOutput, realInput); // compute fft
// fft.completeSpectrum(complexOutput);
fft.fromComplexArray(complexOutput,realOutput); // get rid of the complex value and keep only real


let x =[];
for(let i=0;i<size;i++) x.push(i); //create a simple dumb x axis for the fft plot

plotlib.plot( // plotting the input data
    [{
        x: x,
        y: realInput,
        type: 'line',
        name:'input'
    }]
);

plotlib.plot( // plotting the fft output
    [{
        x: x,
        y: realOutput,
        type: 'line',
        name:'output'
    }]
);

Plotting the input data from the code above give me:

And the output look like this:

While i was expecting to get only one spike, something like this:

1. Is there something wrong with my code or the output is supposed to look like that and i missed something about how to generate frequency spectrum out of an fft?
2. Can fft work with float value right away or it need to be converted to Integer first?
3. Do i need the complex part of the fft output in my case?