Create data reports using javascript function

const reportDefinition = {};
reportDefinition.headers = [report_header, header_level_1, header_level_2, header_level_3, ...]; // default = []
reportDefinition.footers = [report_footer, footer_level_1, footer_level_2, footer_level_3, ...]; // default = []
reportDefinition.compare = (previousRecord, currentRecord, objWork) => {
    // default = () => -1
    // code that returns an integer (report level break number)
};
reportDefinition.display = (currentRecord, objWork) => {
    // code that returns a string, for example
    return `${currentRecord.team} - ${currentRecord.player}`;
};
// source array can be preprocessed, for example filter or sort
reportDefinition.source = (arr, objWork) => preProcessFuncCode(arr); // optional function to preprocess data array
// example to add extra field for HOME and AWAY and sort afterwards
reportDefinition.source = (arr, objWork) => arr.flatMap(val => [{ team: val.team1, ...val }, { team: val.team2, ...val }])
    .sort((a, b) => a.team.localeCompare(b.team));
// optional method 'init' which should be a function. It will be called with argument objWork
// can be used to initialize some things.
reportDefinition.init = objWork => { ... };

reportDefinition.headers = [];
// currentRecord=current record, objWork is extra object,
// splitPosition=0 if this is the first header shown at this place, otherwise it is 1, 2, 3 ...
reportDefinition.headers[0] = (currentRecord, objWork, splitPosition) => {
    // code that returns a string
};
reportDefinition.headers[1] = '<div>Some string</div>'; // string instead of function is allowed;
reportDefinition.footers = [];
// previousRecord=previous record, objWork is extra object,
// splitPosition=0 if this is the last footer shown at this place, otherwise it is 1, 2, 3 ...
reportDefinition.footers[0] = (previousRecord, objWork, splitPosition) => {
    // code that returns a string
};
reportDefinition.footers[1] = '<div>Some string</div>'; // string instead of function is allowed;

// previousRecord=previous record, currentRecord=current record, objWork is extra object,
reportDefinition.compare = (previousRecord, currentRecord, objWork) => {
    // please never return 0! headers[0] will be displayed automagically on top of report
    // group by date return 1 (lowest number first)
    if (previousRecord.date !== currentRecord.date) return 1;
    // group by team return 2
    if (previousRecord.team !== currentRecord.team) return 2;
    // assume this function returns X (except -1) then:
    // footer X upto and include LAST footer will be displayed (in reverse order). In case of footer function the argument is previous record
    // header X upto and include LAST header will be displayed. In case of header function the argument is current record
    // current record will be displayed
    //
    // if both records belong to same group return -1
    return -1;
};

reportDefinition.headers[2] = (currentRecord, objWork, splitPosition) => {
    // this is a new level 2 group. Reset objWork.runningCounter to 0
    objWork.runningCounter = 0;
    // put extra code here
    return `<div>This is header number 2: ${currentRecord.team}</div>`;
};

const reportDefinition = {};
reportDefinition.headers = [report_header, header_level_1, header_level_2, header_level_3, ...]; // default = []
reportDefinition.footers = [report_footer, footer_level_1, footer_level_2, footer_level_3, ...]; // default = []
reportDefinition.compare = (previousRecord, currentRecord, objWork) => {
    // default = () => -1
    // code that returns an integer (report level break number)
};
reportDefinition.display = (currentRecord, objWork) => {
    // code that returns a string, for example
    return `${currentRecord.team} - ${currentRecord.player}`;
};
// source array can be preprocessed, for example filter or sort
reportDefinition.source = (arr, objWork) => preProcessFuncCode(arr); // optional function to preprocess data array
// example to add extra field for HOME and AWAY and sort afterwards
reportDefinition.source = (arr, objWork) => arr.flatMap(val => [{ team: val.team1, ...val }, { team: val.team2, ...val }])
    .sort((a, b) => a.team.localeCompare(b.team));
// optional method 'init' which should be a function. It will be called with argument objWork
// can be used to initialize some things.
reportDefinition.init = objWork => { ... };

reportDefinition.headers = [];
// currentRecord=current record, objWork is extra object,
// splitPosition=0 if this is the first header shown at this place, otherwise it is 1, 2, 3 ...
reportDefinition.headers[0] = (currentRecord, objWork, splitPosition) => {
    // code that returns a string
};
reportDefinition.headers[1] = '<div>Some string</div>'; // string instead of function is allowed;
reportDefinition.footers = [];
// previousRecord=previous record, objWork is extra object,
// splitPosition=0 if this is the last footer shown at this place, otherwise it is 1, 2, 3 ...
reportDefinition.footers[0] = (previousRecord, objWork, splitPosition) => {
    // code that returns a string
};
reportDefinition.footers[1] = '<div>Some string</div>'; // string instead of function is allowed;

// previousRecord=previous record, currentRecord=current record, objWork is extra object,
reportDefinition.compare = (previousRecord, currentRecord, objWork) => {
    // please never return 0! headers[0] will be displayed automagically on top of report
    // group by date return 1 (lowest number first)
    if (previousRecord.date !== currentRecord.date) return 1;
    // group by team return 2
    if (previousRecord.team !== currentRecord.team) return 2;
    // assume this function returns X (except -1) then:
    // footer X upto and include LAST footer will be displayed (in reverse order). In case of footer function the argument is previous record
    // header X upto and include LAST header will be displayed. In case of header function the argument is current record
    // current record will be displayed
    //
    // if both records belong to same group return -1
    return -1;
};

reportDefinition.headers[2] = (currentRecord, objWork, splitPosition) => {
    // this is a new level 2 group. Reset objWork.runningCounter to 0
    objWork.runningCounter = 0;
    // put extra code here
    return `<div>This is header number 2: ${currentRecord.team}</div>`;
};