TypeError: ‘set’ on proxy: trap returned falsish for property

Cached items are set as read-only (because otherwise you could corrupt the cache). If you want a modifiable object, you need to clone it.

this.wiredContact = Object.assign({}, data);

Based on comments, you can also use the rest parameter syntax in LWC:

this.wiredContact = {...data};

If you don't care about some mobile browsers, and you're using Lightning Web Security instead of Locker Service, you can also use the structuredClone function, which can handle even recursive structures and other things that the JSON.parse(JSON.stringify(value)) trick doesn't support. You can check the status of structuredClone on Can I Use.

here is a solution that works perfectly. I display a list of opportunity and add a hyperlink on the column name of the data table. To do that, you have to create a wrapper class in you apex controller like this:

public with sharing class GetAllOpportunities {
   @AuraEnabled(cacheable=true)
    public static List<DataTableWrapper> getAllOpps() {
        List<Opportunity> listOpp = [SELECT Id, Name ,StageName, CloseDate 
                                     FROM Opportunity Order By Name asc];

        List<DataTableWrapper> response = new List<DataTableWrapper>();
        for(Opportunity opp : listOpp){
            DataTableWrapper obj = new DataTableWrapper();
            obj.oppId = opp.Id;
            obj.name = opp.Name;
            obj.nameUrl = '/'+opp.Id;
            obj.stageName = opp.StageName;
            obj.closeDate = opp.CloseDate;
            response.add(obj);
        }

        return response;
    }

    private class DataTableWrapper {
        @AuraEnabled
        public Id oppId {get;set;}
        @AuraEnabled
        public String name {get;set;}
        @AuraEnabled
        public String nameUrl {get;set;}
        @AuraEnabled
        public String stageName {get;set;}
        @AuraEnabled
        public Date closeDate {get;set;}
    }


}

note the nameUrl attribute, I'll use it in the js file.

here is the html file:

<template>
    <lightning-card  title="Opportinity List">
        <lightning-datatable data={opportunities} columns={columns} key-field="Id"></lightning-datatable>        
    </lightning-card> 
</template>

And Finally, the js file:

import { LightningElement ,wire,track} from 'lwc';
import getAllOpps from '@salesforce/apex/GetAllOpportunities.getAllOpps';

export default class OpportunityList extends LightningElement {
    @track columns = [
        {
            label: 'Opportunity name',
            fieldName: 'nameUrl',
            type: 'url',
            typeAttributes: {label: { fieldName: 'name' }, 
            target: '_blank'},
            sortable: true
        },
        {
            label: 'Stage Name',
            fieldName: 'stageName',
            type: 'text',
            sortable: true
        },
        {
            label: 'Close date',
            fieldName: 'closeDate',
            type: 'date',
            sortable: true
        }

    ];

    @track error;
    @track opportunities = [];


    @wire(getAllOpps)
    wiredOpps({error,data}) {
        if (data) {
            this.opportunities = data;
            this.error = undefined;
        } else if (error) {
            this.error = error;
            this.opportunities = undefined;
        }
    }
}

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Update

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This code below does the same thing without creating any wrapper class. We can only use JS.

import { LightningElement ,wire,track} from 'lwc';
import getAllOpps from '@salesforce/apex/GetAllOpportunities.getAllOpps';

export default class OpportunityList extends LightningElement {
    @track columns = [
        {
            label: 'Opportunity name',
            fieldName: 'nameUrl',
            type: 'url',
            typeAttributes: {label: { fieldName: 'Name' }, 
            target: '_blank'},
            sortable: true
        },
        {
            label: 'Stage Name',
            fieldName: 'StageName',
            type: 'text',
            sortable: true
        },
        {
            label: 'Close date',
            fieldName: 'CloseDate',
            type: 'date',
            sortable: true
        }

    ];

    @track error;
    @track opportunities = [];


    @wire(getAllOpps)
    wiredOpps(result) {
        const { data, error } = result;
        if(data) {
            let nameUrl;
            this.opportunities = data.map(row => { 
                nameUrl = `/${row.Id}`;
                return {...row , nameUrl} 
            })
            this.error = null;
        }
        if(error) {
            this.error = error;
            this.opportunities = [];
        }
    }
}

Apex class

public with sharing class GetAllOpportunities {
   @AuraEnabled(cacheable=true)
    public static List<Opportunity> getAllOpps() {
        return [SELECT Id, Name ,StageName, CloseDate FROM Opportunity Order By Name asc];
    }

}