can't access data variables in watch handler vuejs

Question

I'm trying to set a data variable in a watch handler function for an input field in a VueJs Component. I have something like this:

data() {
    return {
        params: {
            // default params to 1 month
            from: new Date().setMonth(new Date().getMonth() - 1),
            to: Date.now(),
            skip: 0,
            limit: 100
        }
    }
}
watch: {
    dates: { 
        handler: date => {
            console.log(this.params)
            if (date.start) {
                this.params.from = moment(date.start, "YYYY/MM/DD")
            }
            if (date.end) {
                this.params.to = moment(date.end, "YYYY/MM/DD")
            }
        },
        deep: true
    }
}

When I set an input for the dates variable in the view template, I get an undefined for this.params in the console log, and I get an error for trying to set this.params.from. So I tried accessing it using a method:

methods: {
    printParams() {
        console.log(this.params)
    }
}

calling it in the view template, it correctly resolves the params object.

Am I missing something here?

Answer 1

To avoid additional binding, just avoid using the arrow function syntax here.Instead go with ES6 Object shorthands:

watch: {
    dates: { 
        handler(date) {
            console.log(this.params)
            if (date.start) {
                this.params.from = moment(date.start, "YYYY/MM/DD")
            }
            if (date.end) {
                this.params.to = moment(date.end, "YYYY/MM/DD")
            }
        },
        deep: true
    }
}

Now this will be bound to the correct context by default.

Answer 2

Let's try bind this to your handler

        handler(date) {
            console.log(this.params)
            if (date.start) {
                this.params.from = moment(date.start, "YYYY/MM/DD")
            }
            if (date.end) {
                this.params.to = moment(date.end, "YYYY/MM/DD")
            }
        }.bind(this)