How to set default value in Combo box in Lightning Web Components(lwc)

To set the default value in the Combo box assign the value in js code directly.

Ex:

HTML Code

<template>
    <lightning-combobox
            name="progress"
            label="Status"
            value={value}
            placeholder="Select Progress"
            options={options}
            onchange={handleChange} ></lightning-combobox>

    <p>Selected value is: {value}</p>
</template>

JS Code

import { LightningElement, track } from 'lwc';

export default class ComboboxBasic extends LightningElement {
    value = 'inProgress';

    get options() {
        return [
            { label: 'New', value: 'new' },
            { label: 'In Progress', value: 'inProgress' },
            { label: 'Finished', value: 'finished' },
        ];
    }

    handleChange(event) {
        this.value = event.detail.value;
    }
}

Output